Muck or water in the fuel could be the problem. In theory the filter in the fuel pump should prevent debris getting to the carbs, but fine particles still get through. Suggest you take the tops off the flat chambers (assuming you have SU carbs), mop out the contents of the float bowls with an absorbent cloth and see if there's any crud in the bottom. It's worth toggling the lever on the fuel pump at this stage to check fuel flow. The fuel pumps have non-return valves in them. I had an instance of these failing intermittently so the fuel feed to the carbs was erratic. It was hellish frustrating until diagnosed.
In some of the pumps these valves are secured with a screwed in plate; in other pumps they are fitted and the pump casing is peened over so the valves cannot be replaced. Were yours the replaceable type?
Other than that it's electrical, but it's odd that the engine revs fluctuate. But you could check the points gap, fit a new condenser maybe and fit a new coil. The problem with the coils on TR engines is that they bolt to the engine block, so they get very hot and can fail prematurely. This happened to me. I now mount the coil on the inner wing.
Please let us know how you get on.
Tony
