This really sounds like a damp problem. The rough tickover coupled with damp would indicate shorting in the HT circuit. As simple test is to put the car in complete darkness and start it. Look along the HT leads for little blue flashes. You would be surprised what a firework display can be seen. DONT TOUCH THE HT LEADS!!!!! unless you want your head imprinted in the bonnet!!!
740/940s were suseptable to shorting if the leads either touched each other or went near metal...i.e. the brake pipe from the master cylinder. Simple answer is to separate with plastic spacers...cut up a washing up bottle and make grouves in it. Slightly more expensive is to replace leads as the carbon brewaks down with age / pulling / vibration.
Also try cleaning up the insides of the dizzy cap. This tends to be ignored as its not easy to get off.
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