gwen
On Vista, W7,W8 there is something called the driver store. WHen you install a USB device the driver is put into the store. I have encountered situations where multiple drivers for the same USB are contained in the store. If the manufacturer properly writes a new driver, when it is installed it replaces the old one in the store and there are no problems. Unfortunately not all manf's write correct drivers. I design USB devices and have to deal with this constantly and it causes all kind of problems.
If you have installed the same driver multiple times or updated to another driver without uninstalling the old one, it might be a good idea to get rid of ALL the old ones. 
Microsoft does not make this easy but there is a rather easy way to tell if multiple drivers are installed.
Open a command prompt as admin.
type in
pnputil.exe -e
hit enter
You will get a list of all  drivers in your store. There may be like 30+ depending on your system
 
Here is just two fro my laptop.

Published name :            oem5.inf
Driver package provider :   E-MU Systems
Class :                     Sound, video and game controllers
Driver date and version :   10/07/2010 6.0.01.0001
Signer name :               Creative Labs Inc

Published name :            oem1.inf
Driver package provider :   Realtek Semiconductor Corp.
Class :                     Sound, video and game controllers
Driver date and version :   07/28/2009 6.0.1.5904
Signer name :               Microsoft Windows Hardware Compatibility Publisher
 
Find Tascam in the list then see if there is more than one Tascam listed.
You can scroll up/down in the command window.
You could just post it if you want and I will take a look.
If there is, then it might be worthwhile to make the effort to remove all the dupes and
start fresh. This is kind of a pain and is why I posted the above. But I can point you to the process if need be.
 
Michael
 
 

... you know, the USB cable was a mini-USB cable, the only one I had, and it was off of a garmin GPS... it's probably not even 2.0......  and that factor IS the same between the two computers...

i'm gonna track down a new one, and hopefully follow it up with a giant order of humble pie.
 
even if its just a USB cable (im praying), this thread has some of the most technical responses i've seen in a while, even with all my digging in to what seems like every issue i can have and then some the last week or three.  the fact that any of you were willing to get this detailed about things just for some no-name wanna be producer girl in idaho, and let alone keep trying with me after the thread isn't new anymore, and the simple answers have been exhausted really speaks to this community (ignoring for the moment the fact that it might be the simplest of answers... wrong usb cable xD).  whenever i finally get up and running again, this experience has probably cemented me as a lifelong cakewalk user.  this community is amazing :)
 
-gwen

gwen... FYI, my Tascam driver is indicated to be version 2.1.4.0 in my computer Device Manager.

You can thank us when it's fixed!  I sure hope something we've already recommended gets it going 'cause I'm scrapin' the bottom of my barrel o' tricks. 

dang!  i about jumped out of my chair.  my device manager shows the right driver, I thought you meant the tascam control panel at first where it shows 2.01.  Thanks a bunch though Tony :)  hopefully I won't have to bug you anymore!
 
-gwen

Interesting... I just opened my Tascam control panel, and it says Version 2.01.  LOL
 
Not bugging me at all my friend.  Just hope we get your problem fixed.  Please go get a new USB cable now.  LOL

Welp, USB cable made the latency a little less but I'm still seeing variability.  The 25ms latency I have over what the control panel says it should be (I imagine that is monitoring latency and it would be roughly double to bounce it back again through the mixer and to the computer).  Interestingly, now I'm showing identical latencies on both my recording computer and my laptop (30.17ms on a 4.35ms setting in the tascam control panel).  I guess my next step is to install the DAW on the other computer and see if I get the same variability when bouncing back my metronome.
 
This computer was sitting for a couple years while I was away from home for work and I do seem to be having a small bit of trouble with some things on it (a certain USB wireless mouse works fine on my laptop, intermittently on my boyfriends computer, but not at all on this one -- also a fresh install of windows is giving me long file-access problems as if my harddrives have gone a bit bad).
 
If you get the chance Tony, could you run that CEntrance utility and tell me what it gives you?  Maybe let me know just how you're routing the signal back to your computer, too, I might be adding extra jumps I don't need.
 
Thanks again guys :)
 
-gwen

oh, and if Michael happens back across the thread, I just want you to know I did try your suggestion.  The list is pretty clean.
 
-gwen

here is an example of my recording variability;   imgur.com/lZEB1OF
 
 
First track is straight recording metronome, the rest include track one so I can see the difference, if any, between straight computer bounce back and replayed audio bounceback.
 
I've also tried it where I silence the previous tracks instead of muting them (maybe muting keeps the computer from ever loading the sample, while having the gain turned inaudible would still add for the factor of computer stress?)
The result is about the same:    imgur.com/uQ65e18
 
After doing this countless times in different ways, it does seem that there is a general trend (though any one image like this would give a wrong impression as to how frequent or infrequent it is) of the latency being its shortest with the first track, having a few that are more longer but identical, then getting longer as more tracks are added.  However, this is not consistent, and they still tend to fall in the exact same places.  Interestingly, after an audio dropout (they're rare but do happen), as seen in the last recording of the first image, the track returns to it's lowest latency even with all of the other tracks present.  Usually.  Even the first track isn't very consistent.  It almost seems as if the system is building up stress which causes the triggering of the latency to get more likely.
 
Hopefully this will not be repeatable on my laptop.
 

gwen,
You mentioned silencing? then muting the unused audio channels.
 Have you tried disabling the unused channels in AUDIO DEVICES?
From the Tascam M-164UF manual it looks like you have 10 channels + 2 aux.
I have a Tascam FW-1884 (firewire) and although I have not encountered your issue, I have found that disabling unused channels does affect audio quality at higher sample rates.
This makes sense since the available bandwidth of the interface is being divided between the channels.
As a precaution, disable all unused MIDI channels as well.
Since you do not seem to have a multiple driver issue, I am running out of suggestions.
THe manual mentions firmware updates, have you looked at this. This is critical with my FW-1884.
 
Michael