njon, I can look into this but I need to make sure we are on the same page.
What version of Sonar are you usng?
Which DimPro version do you have( I am on 1.5).
What is your sound card/ interface? Are you using ASIO drivers?
What is your MIDI Prepare buffer set to?
Are you using a drum map?
I am also using XP(specs below).
I have also been successful with other synths(EZDrummer, Session Drummer2, Studio Instruments) but have not tried triggering DimPro.
I'll get back to you with what I find out.