Problem solved!! As suggested the problem was a connector somewhere. I ended up pulling all the connectors to and between the MPU and driver board and also the joiner ones between the playfield and the MPU - turned it on and everything worked as it should. Thanks again for the help.
I also put a very small amount of WD40 on the connectors...unsure if this was a good idea or not.
Rob - Nice one
BUT - All you have done is Identified the problem and bought some time until it happens again.. You will need to replace those connectors in time.
There's an entire section devoted to the interconnectors here;
http://www.pinrepair.com/sys37/index1.htm#interconnectUse a pencil eraser to clean the connectors - Never use WD40
