I should have mentioned the PSU outputs as labelled on the actual PSU unit, so here they are:
Its a 350W ATX Allied Model AL-C350ATX:
+3.3v = 28A
+5V = 28A
+12V = 19A
-12V = 0.8A
+5VSB = 2A
Could this be the problem - underpowered PSU? Also, would I need to replace the CMOS battery, even though its not reporting any error messages?