I think I'd be looking for an air leak in the carb/induction area. Try spraying WD40 or brake cleaner around - it'll get sucked in if you spray the leak point and it'll alter the revs alerting you to the location.
Also maybe fuel starvation - is fuel getting throu ok? A possible culprit is a rubber hose breaking down internally - that would be worse when hot. I'd be looking at the fuel line in the engine bay area first. Float level could be suspect as suggested also.
I have had some dodgy electronic components before - if you had a condenser I'd be swapping that out to check. Not so easy with the electronic kits but if all else seems ok on the fuel side I'd be suspicious of the little box of tricks hiding in the distributor.
Good luck searching.
