Assuming it's being caused by ignition breakdown, there's quite a lot it could be but I'd start by checking the following:
- spark plugs. Are they in good condition and the correct ones (Bosch W5DPO or equivalents).
- ignition coil. If it's an old one then it's a good idea to replace. It's the same coil used on loads of vw group engines such as 6n polo so they don't tend to be that expensive
- hall sending in the distributor, check the condition of the connector. You can replace this part without replacing the whole dizzy, just be very careful getting the roll pin out and in of the plastic wheel. Alternatively, get a dizzy from a multi-point injection 6n polo.