Potterton 40e Netaheat 3A Spur Fuse Blowing?

We have a Potterton 40e Boiler and it has worked fine for 6 years. In the past few week it has resorted to blowing the 3A fuse to the Programmable control unit. Now the strange thing is I can put a new fuse in and it will work for anything between 1 and 3 days before it blows again. I have checked the wiring to all units and they are snug and tight in the conenctions.
The system is an ACL 805 Switchmaster, ACL Mid Position, ACL Cylinder Thermostat and Danfross Pump. The programmer is set to Water Constant and Heating Timed. It would appear looking at the programmer when it stops that the fuse blows when its water only. Does anyone have any idea's as to where the problem lies as I have spoken to ACL Drayton - Invensys and all they can tell me is that I need a voltmeter to check it out but if I don't know where the fault lies or when it is likely to happen how do I know what to look for.

Is there enough power in a Motorised Valve to blow the fuse?

Is it that there is a fault with the Boiler in that a Fan is sticking that could cause the fuse to blow?

Any idea's. Electrically minded but no idea with regards to plumbing. As mentioned before ACL Drayton - Invensys could not help me and the only option I have at the moment is to replace all the items to see if the problem goes away.

Russ Hounsome
December 2006