Do a search on the topic!!!!! What year is your car? How many miles? What about the cables and coil? Did you jumped your car recently? If you did, I would check the overvoltage relay fuse. It is located behind the batttery, it a silver cylindrical box with a red plastic cover/top, it has an "american" type fuse. Are you sure the cap and rotor were replaced?
Start with these simple things, and go from there...
Good luck!!!!!

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J.H.
'86 300E