jumpy playback

Catalyst V4 m38 ProHD (10 layers / 1 VGA output, 1 preview output & 1 pixel mad output)
Artnet - Ether-Lynx (2 DMX outputs to LED fixtures)

I have a problem with catalyst.

After a while playback will start stuttering or stalling slightly. The bars at the bottom of the screen will jump over 100%.

The system does not need to be under load for this to happen.

Restarting solves the issue for a while...

Anyone seen this before??