Buick LeSabre No-start Problem

Hi Guys,
My LeSabre will idle very roughly for weeks but will start without a problem. Every few weeks after being driven for several miles and at operating temperature it won't restart for half an hour or so. It
cranks OK but just won't start. When it finally does start it runs a lot smoother and seems to have far more "pickup." (Another possibly unrelated issue is a very loud metallic bang noise that can be felt and heard towards the middle of the car. This usually happens while idling at a stop light or a few seconds after shutting it off. I can actually feel it in the floorboard.)
This car has had several new PCMs, coils, plugs, wires, sensors, alternators, EGR, PCV, etc., etc., since I bought it two years ago. I took it back to the shop this evening because they couldn't figure out what was wrong last few times and left the ticket open. The shop called today to say that the fuel system checked out OK as did the ignition system but they think it is ignition related. They just don't know where or what.
Meanwhile, I've practically rebuilt this car. Does anyone have any suggestions? I'm at wits end. It's a very well maintained 1994 Custom with under 92,000 miles. A friend mentioned the Pass Key Anti Theft system,, that maybe the key has gone bad but that doesn't explain the idle smoothing out after a no-start or the noise.
Thank you for your help!
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MidwestBuick wrote:

Obviously something is going bad due to heating up.
I would check the coil packs first, then move onto things like the EGR and so on. Check cold and then hot - if the coil(s) are showing big changes as they get hot, then there's your problem.
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MidwestBuick wrote:

That much sounds like the crank sensor. The other symptoms could be anything: fuel injection, cam sensor, etc.
---Bob Gross---
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The 'bang' suggests to me that maybe you're getting fuel burning in the catalytic converter. Given the age, I'd be looking at the O2 sensor (for the over-richness) and the converter (probably damaged by the combustion).

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