Disappearing softsynths

 Often after I create a new project and I import some midi tracks, I find that the softsynths don't work. Then sometimes they'll magically start working. On this particular project, they will fire the first midi note and then go silent. I can see that they're still playing by the volume indicator for the track, but there is no sound. On this particular project, when I hit a note on my keyboard, it will sound but only after about 5 seconds. Normally the latency is high but no more than a half second. All the audio plays normally, and the midi plays normally if I output to an outboard synth. It's just the softsynths that don't work. In the event viewer the midi tracks seem normal. X1 on XPsp3 2.66 quadcore Tascam US-122 usb audio/midi module 2GB ram 2 10k rpm HDDs softsynths are Dimension Pro and Kontakt (Steven Slate drums) What are the most likely culprits to look at?