Engine starts runs ok until it gets upto temperature then everything goes haywire
i.e. eml comes on, rev counter, fuel gauge & temp gauge go crazy, misfires, and eventually cuts out. It will start again with alot of trouble if you keep revin it up. If I let the engine cool down for approx 10mins it starts ok but feels rough with no misfire. It will rev until back upto temperature then the whole cycle starts again.
Had the code read and it came back as Dispack so changed the pack with GM Dispack went ok for 1/2 hour but faults back.
Anybody have any ideas please?

Take the lid off the trunking that runs along the side of the engine, and have a good dig around looking for any wires that have chafed/shorted to others.  The wiring from the ECU to the DIS runs down there.

Was the DIS that you took off physically damaged in any obvious way (casing split etc.)?

Edit, oh and use a paperclip to see what codes the ECU is currently reporting, if any, for free!
See maintenance guide index, Paperclip Test.