Rev counter bobs up and down and the lights go with it

dear andy
i don't know your level of exp with car electrics but here is my theory on your problem. usually there is a small voltage regulator/stabiliser situated on the dash p c b, which sometimes gives problems like yours, in which case it will need replacing ( that is if g.m.still use them ). depending on your level of car knowledge, if you don't know a lot then get a auto electrician to do the checks/repair for you. however if you feel you want to try then get a manual & look at the wiring diagram section & components to locate the reg, i must stress that i've not worked on vauxhalls a lot, but the early ford voltage regs were held in place by a small screw which when removed the unit just pulled out & new one pushed in ( made contact by 2 fixed male spade lucars). anyway best of luck andy, let me know how you get on . regards dj dave