Alfa spider 2 litre 1993 S4

Just got the car 3 months ago.
After using it for a few weeks it wouldnt start suddenly, thought the battery so replaced it. after a few weeks the same thing happened. The engine would not turn over but all lights on on dash as normal. No noise of starter problem or engine turning just silence.
checked out all electrics over the battery and found all readings satisfactory. Car always starts right away with a jump
Today wouldnt start checked battery and it was flat
no drain of power showing. checked earths and all ok
starter and alternator seem to be working fine. replaced battery to take old one away to rule out a bad battery
power readings perfect over battery when ticking over and at 2000 revs.
Ruled out bad starter as rocked it in 4th and tapped with hammer and still wouldnt start, anyway a jump shouldnt help if starter jammed.
should I turn my thoughts to a resistor at coil problem if there is one, or the ignition system?
all help appreciated

the problem could be in the altinator, even though it puts out enough charge when running it could be grounding out when the car is shut off. you say it starts every time with a jump. when it happens and the car wont start are the lights come on bright or low as if the battery is low? you can take it to a auto store and have the charging system checked and they can also check to see if there is a drain when the car is off.

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