1993 2.0L manual wentworth turbo UK

Picked up a new problem yesterday where the car started running a little rough at lower speeds but just managed to limp home. Seemed to be having trouble with ticking over.

Left overnight and cleaned iac controller this morning (it was very clean surprisingly) and checked vacuum hoses (all good bar one which I replaced)

On startup, thought it was better till I blipped the throttle then it started surging the tickover up and down all on its own! I stood and watched this for about 8 cycles, noting that it dropped the revs almost to cut out then brought them up again to about 1200 revs. The only visual fault I can see under the bonnet is a crack on the distributor cap where the top mounting screw goes (i can see the metal but theres no arcing going on) and the usual audible clack clack clack from the leaking exhaust port gasket ive been meaning to fix for years. (ive now bought the gaskets)

Also, I just tried a different Fuel pump relay which made no difference to the behaviour, but I did note interestingly that on startup the engine starts and idles at about 700rpm absolutely fine for about 20 seconds then starts hunting up and down.

I would be most grateful for any pointers on how to get going on this mystery. Im used to it being a bit of a struggle but any shortcuts would be very welcome.

regards