Could be a number of things....
Firstly are the spark plugs good? Have they been replaced on a service in say the last 15,000miles or so? If not replace these first. (About £10 a set)
If the spark plugs are OK first make sure you have good connections to the coil, the main lead to the distributor as well as the smaller wires supplying the coil (remove wires and clean if they are coroded). There is also an earth strap between the gearbox and the chasis on the passenger side of the engine bay, make sure this has a good connection too and is not coroded.
After that try cleaning the contacts in the distributor cap or replacing the rotor arm and the distributor cap, they cost about £20 for both and are common problems on older cars.
If thats no good try replacing the plug leads and coil leads as these are fairly low cost too. After that its probably down to the ignition module/ECU.
Matt, December 2009
