Why won't it start after sitting a few hours then start OK the next day? on 1994 Volvo 960

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Replaced Ignition Control Unit and the Ignition Control Module and the problem still exists. The ICM fixed it for a few months. Now It will run fine for a day or two then after being parked for a few hours won't start. Quite often it will start the next day and then run fine for a while. Two different mechanics have attempted to fix it but the problem is sporadic. The mechanics think it is not getting fire.
(1) Answer
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Not super familiar with your vehicle, but here's something to think about: On many vehicles, the ignition modules are mounted to a machined flat spot (usually made of aluminum) on the distributor, intake manifold, etc.. If this is the case with your car, It is VERY critical to apply a good amount of dielectric grease between the module and the machined spot where it mounts to. This is done to aid in cooling the module (the heat transfers through the grease to the part the module is mounted to). If your car requires this and the grease was not applied, it can ruin a new one relatively quickly.