My Olds 88 has 139K miles and for the past year I have had a problem of the engine not starting. This happens maybe once every week or two. It will start in the morning to go to work but not start in the afternoon. Engine cranks good, all lights work, no code on the dash, but won't fire and start. Let it sit for 30 to 45 minutes and it will sometines start without any problems. This happens with no reason. It may run fine for days or weeks then just won't start.. It has been in the shop numerous times and can't be diagnosed when there isn't a problem. It's like a grimlin decides to keep it from starting for no reason. Sometimes it will start in less than an hour of letting it sit and some times it takes numerous attempts over a period of 8 hours before it will start. After leaving the car sitting in a parking lot overnight when it wouldn't start, it will start on the first try the next morning. There has been times when the car would just die when driving 5 to 10 minutes after starting, but would always start on re-attempt. I have taken a screwdriver handle and tapped on various parts under the hood in frustration and the engine would then start, then when it would happen again the tapping wouldn't work. Two different GMC dealers haven't been able to shed any light on the problem. Their answer is "if we can't replicate the problem, then we can't solve it". Once we had it towed into the dealer because we couldn't get it started and when the tech got in, it started right up like there was no problem. I like th ride and power the car has, but the dependablity issue is getting in the way.
Has anyone experienced this problem and how did they solve it? I'm wondering if there is an intermitten computer problem.
Help please.

Dave