Car is a 1978 Triumph Spitfire 1500. Pretty standard except for electronic ignition and 12V coil. The battery is in good condition.
When cranking the engine on the starter, most of the time it will work fine. Occasionally, it will crank very slowly (as if the battery was flat) then pick up to normal speed again. Sometimes this doesn't happen at all, other times it'll crank slowly for ages eventually reverting to normal operation (which can then start the engine). There seems to be no telling when it'll behave like this.
I thought at first the alternator was playing up and trying to charge the battery when the car was being cranked. I've since changed the alternator with no improvement. I have checked thoroughly the wires from solenoid to starter, and from ign. switch to starter and all seem well. I think this is a coincidence, but it started doing this shortly after I bypassed the ballast resistor to install the 12V coil. Any ideas?
I appreciate that this isn't much to go on, but I wondered if this was a 'classic symptom of starter motor/solenoid/ignition switch failure' etc?
Many thanks, David