I had similar problems and changing the diverter valve to a decent qualitry valve (Honeywell) sorted it out. A fairly straightforward DIY job once you've drainied the system down.
Note that some Potterton boilers around that time appered to suffer with with their PCBs. Potteron now offer a replacement kit - for about £230...
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.