If I'm running on GAS then switch over to petrol, it will run ok for about 5 mins then it will completely cut out, and lose all power. Then it won't work again until it decided is wants to!!!

erm so yeah...debugging this is a bit tricky. its' a 2.1i DJ 1987.
I've recently replaced -
leads,
distributer cap,
rotter arm,
plugs.
air filter.
I took it to the MOT station today (on petrol) and it cut out and wouldn't even start again on petrol or gas

so yeah any pointers on trying to find out why it cuts out randomly would be much appreciated

Cheers