Your computer should be smoking along without any issues. That is 4x more a machine than I have.
I think if you mess with the setting that things get buggered up.
I have never adjusted the latency settings or buffers so cannot comment on which is best, I just let Sonar set this up and it has always worked perfectly for me.
There is nothing wrong with using the WDM drivers. As a matter of fact I did a little reading and came to the conclusion that for some PC configuration ASIO drivers, which are not native to MS systems, might not work as well as the native WDM drivers. We are even told by Cakewalk themselves to try both and use which ever seems the most stable.
Just a note on the 3/4 inputs. Under ASIO on some they will share drivers with the SPDIF and you have to select either SPDIF or 3/4 as an input in the Tascam control panel.
Try setting the Tascam control panel to the second lowest setting, not the lowest.
There is a test to see if SOnar is not adjusting for latency offset.
Use a midi kick or snare drum track.
Bounce it to an audio track
Or use a real snare or anything with a super obvious transient spike. Hit your guitar PU's. anything.
Patch your output of the Tascam back to a input and re- record to a new audio track.
Put them side by side and see zoom in to see if the transients line up.
There is a way to adjust the offset manually.
But I think there's something gone wrong when you have to do this as Sonar is normally dead on.
Are your monitoring from the Tascam and with input echo off?