What Tom and I were mentioning about using other driver modes is important. You might not "hear" your audio tracks being out of sync but unless you don't mind sloppy mixes you might want to test for offset timing issues.
You obviously ARE dealing with latency as you mentioned the changes when you lower your buffers.
To perform a loopback test:
Have a midi kick or snare drum track with a strong velocity
Use SD2 and freeze it to create a audio track. It should have nice big transients that will line up perfectly with the midi notes if you zoom in.
Now loop this back to a new audio track by patching your audio interfaces output back to an input. Make sure input echo is OFF.
Record the new track and zoom in to see how the transients line up with the original.
You can clearly see in my screenshot how ASIO is perfectly aligned and the other modes are late.
I did this loopback test with 4 different computers and using 3 different ASIO audio interfaces as well as a few none ASIO interfaces. The results are always about the same. Interestingly ASIO4all will be perfectly in sync when using an on board sound card. My conclusion ( and many others too) is only ASIO should be used for overdubbing audio tracks. The workaround is you can manually adjust the timing offset but what a PITA that is.
And your issue could very well be a combination of things due to outdated VST GUI or a slow video processor.