Hi Chris, thanks for your reply.
The sound card is an M-Audio audiophile 2496.Input Latency is 6.531ms and output 6.485ms.Im triggering my external Roland drum module.
If I set it all up and just play the drums through the system. i.e. local off. MIDI output to computer, thru Cubase (input and output level meters ok) back to drum box and then headphones it sounds fine. But when recorded the playback is all "jerky". Ive found another strip of RAM in a machine at the office so now I have 512megs. So that must be enough.If I record the drum module as an audio track it is also fine.Its as if it cant process the MIDI info fast enough somtimes when writing to disk.