Has this only started happening recently on damp mornings? If so then it is likely to be a high tension ignition problem.

Ignition leads(probable cause), coil pack (or maybe coil/distributor cap/rotor arm at this age?) The damp autumnal mornings could be causing high voltage leakage.

N.B. Old VW Polos (same engine etc..) were very prone to this as the distributer cap was positioned just behind the radiator. Every year approaching winter I used to put the leads/cap(after cleaning) into a low temp oven for a few mins (on greaseproof paper lined baking tray) and then refit whilst warm smearing grease (not copper!) over them including connections. This used to protect them from road spray and condensation.