Connect a voltmeter to the battery while the problem is happening. If you see the voltage fluctuating then you have a bad regulator/rectifier on your Alternator.
Otherwise it could be a bad ignition coil.
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'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery
86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)