Yep, my first suspect would be the coil - or if you are still on points ignition - a duff capacitor in the distributor. Yes, I know your mechanic will have just changed the capacitor for new; that's why I suspect it. There have been some duff batches around recently that misbehave straight out of the box. Cheap enough to try, anyway; or better, use an old one from your magic box of spares that lurketh in all good garages!
After those two, I too would be on to fuel starvation. With a recent re-commission, the first port of call would therefore be the cooling system. Spend a long time flushing out the radiator with a hose pipe on full bore, then move to the engine for the same treatment, and finally spend a while with the hosepipe running through the heater. A nice quiet days entertainment for a hot day with the grandkids! It may be as well to be prepared with a full set of coolant hoses, as you will disturb most and old hoses don't like that!
When done, don't put any antifreeze in the system just yet, there are a couple more checks to do. First check is to see if there is water flow into the top of the radiator from the pipe that runs from the "tower" between the two carburettors. You should be able to see this with the help of a very little engine revs with the cap off the radiator. If not, then this is almost certainly the primary cause of fuel starvstion. Take the hose off the "tower" and unblock the hole into the manifold. Some serious aggression with a hammer and a jewelery screwdriver or equivalent is likely to be needed.
Second check is to inspect the thermostat by removing its top cover casting. There is some chance of snapping a stud here, so don't force things too much unless you feel up to coping with the consequences. The thermostat should have a pilot hole - about the size of a 1/8 drill bit (clue for if it hasn't!) positioned at roughly 12 o'clock as you look at the stat. It might even have a little dumb bell shaped "jiggle pin" through it. If not change the thermostat, or if you are confident it is working properly, drill your own hole.
Now you are ready to finally refill the system, including the antifreeze. When full, run the engine up to temperature with the radiator cap off. When the top hose to the radiator gets good and hot, demonstrating that the thermostat has opened, rev the engine moderately for a lengthy period. Then top up the radiator. Repeat this until you are quite sure that all the air has been purged from the system. You might well take 20 mins to 1/2 hour doing this. Then top up the radiator finally at idle and fit the cap.
Report back to us how the car now performs and we can sort out any other attention required from there.
If you don't feel up to this yourself, print this out and give it to your mechanic.
Hope that helps!
"Lucky", '70 3500 (4.6 and ZF in waiting)
"Baby Rover", '89 216 VDP Auto 30k mls