Gideon K
Anderton - The RME manual mentions SONAR specifically as using a WDM system and I've been using my soundcard in WDM mode for almost two years without a problem. I'm pretty sure it's simply to do with buffer size/bit depth settings and that sort of thing.

 
First, did you check Device Manager to see if other drivers were installed? In particular, check for alleged "HD" audio drivers as described in Week 35 in "Friday's Tip of the Week." 
 
The RME manual is right, SONAR can use WDM. However SONAR can also use ASIO. In all of my experiments with audio drivers and dozens of interfaces over the years, ASIO on average gives better performance. The only real advantage of WDM is that it can aggregate interfaces. This is why it's important to check what other drivers are "co-habitating" with the WDM driver you're using, if you insist on using WDM.
 
If you've updated Windows or your computer lately, added drivers (or re-enabling drivers that had been disabled) could enter into the picture that would interfere with performance. 
 
It's a simple matter to select ASIO in SONAR's preferences and see if that offers an improvement. But, also check Device Manager as I advised originally because "rogue" drivers can degrade ASIO as well as WDM performance.
 
FWIW WDM was a noble experiment to try to reduce latency with native Windows drivers, a la Core Audio for Mac. It never really got much traction; several interface drivers don't support it at all. Unless you want to sit around for a couple years, which is the last timetable I've heard from Microsoft for an internal audio engine protocol that can rival Core Audio, I highly advise using ASIO. 

Gideon K
Sonar doesn't have any listed recommendations for buffer size at other bit depth levels than 16 and 24 though. Not that I can see.

 
There can be no recommended buffer size because it's so dependent on the computer itself. I assume in a previous post you meant 256 sample buffers, not bits. That seems high for an RME interface, as they have great ASIO drivers. Many people in this forum report using buffer sizes way under that with RME interfaces using ASIO. This is another reason I suspect you may have "unwelcome guests" in Device Manager.
 

One other thing - when you get the dropout notification, do you click on the Help button and follow the various steps to see if one of those remedies solves your problem?

Ok, I've switched to ASIO. Seems to be working fine. I think I've noticed a genera improvement in the playback sound quality. Is it really necessary to uninstall the realtek sound card? I also use this laptop for non-music production things.

I've been into device manager and disabled the Realtek High Definition Audio under sound, video, game controllers, rebooted. It stayed disabled.
 
Got some more recording to do today so will report back if the problems persist. Many thanks for all the help guys.

Gideon K
Is it really necessary to uninstall the realtek sound card? I also use this laptop for non-music production things.

There is generally no need to uninstall the soundcard driver. Disabling onboard audio in device manager works just fine on most laptops.

Yeah I would just disable it. If for some bizarre reason you end up needing it later you it's just a reboot away.

I disabled it. Still getting problems though.

I started out today trying to record some guitars. Current settings at 44.1/24bit - shouldn't be taxing the system too much.

I record about 10 tracks of guitars, some I keep, some I delete, stop/start, working on amp sounds, mic placement etc. I decide to scrap the earlier takes and save them in a session then start a newer version of the track with those earlier takes not in it. I'm trying to get a full-take of the whole song, which is about 4mins 30 seconds and it starts giving me dropouts. Each successive take gives me a dropout at earlier and earlier points in the song.

I'm trying to follow the help guides, but I don't know where/how to begin.

This applies to all USB2 interfaces....
 
Avoid using USB3 ports (Theoretically, they're backwards-compatible, but in reality, they can be flaky with USB2 devices).
 
Try different USB2 ports, as they are not created equal (ie. If you're having problems with a USB2 port in front, try the ones on the rear panel, or vice versa).

Are you tracking with all your mix/monitor effects enabled ?
If so try this...
Hit the 'E' key to toggle effects to off.
Start with your buffers set to 256.
Record anything.
If no dropouts then your effects are loading your PC then try the following...
Keep lowering the buffer size and recording until you get dropouts. Double the buffer size.
This is your recording buffer size.
Any effects are not recorded so during tracking any effects are in the mix/monitor path only.
You can also selectively power off VSTs one at a time and see.
For mixing, enable your effects, increase your buffer size to 1024 and verify you are good with effects enabled.
This is your playback buffer size.
 

I don't have any monitor effects as I'm using a version of TotalMix which doesn't support/have them. All I'm hearing of the recorded signal is what the mics/preamps are hearing. I don't actually know how to record on Sonar so that internal effects from software are printed onto the recorded tracks. I either record with the original signal generating the sound, or I add plugins after I've recorded.

On the playback in my software, I'm using very minimal effects - some compression for levelling and balance, but using very low-cpu usage plugins like the sonitus compressor, and some hi-pass filtering on the pro channel eq strip. I like to record with external effects and processors about 50% of the time and pedals/rack units play no part in CPU/Sonar performance.

I'm going to try the buffer size tip you suggested.