Recording problem - playback degrades to noise

[Zozobra]

After maybe 30 seconds to 1 min or so playback is just completely falling apart.

Does anyone know how I can fix this?

Using reaper with its onboard effects (the track has 20 in total) and superior drummer, 24/44.1

PC is i7920, 6Gb of 1600mhz CL9, WD caviar black 1TBs and a FF800. I somehow doubt that the problem is in any of these; its a high spec machine and a good interface, but if anyone knows better please correct me. Bitrate on the FF is set to 128 and I'm getting 3.9/5.0 latency.

Total CPU usage is about 13% peak, so its not that.

The HDs are about as fast as it gets without using SSD. There are somewhat faster RAMs and CPUs, but not by much.

It is worse, however, on my other, inferior machine.

But then, one is using vista 64 (the lesser rig that its worse on) and the faster one is on windows 7 64. Reaper 64 as well.

Any suggestions much appreciated; I'm at a complete loss.

Edit - its mostly the drums that are fucking up btw.

 

[Zozobra]

Oh, if you start playback again from any point it resets and sounds fine, then does it again a similar amount of time later. Increasing buffer size has some effect but not much.

And it was all fine for ages until I put a few effects on it. I took them off again and it stayed fucked.

And while the whole thing just sounds like a huge mess, if you solo one track, its a very noisy time and pitch stretched sound, like its running on the wrong clock (which it isnt).