Logic Pro 9 System overload

Using Logic 9.1 on a 2 x 2 GHz Dual-Core Intel Mac pro with 3 GB RAM (1st generation).
Occasionally playback stops (not using excessive track count or plug-ins) and the message 'DISC IS TOO SLOW OR SYSTEM OVERLOAD -10008'. Any thoughts ?
It was the first generation of Intel Xeon Mac Pros (late 2007), and is the slowest of the quad-core range (I believe).
Best wishes