Buick LeSabre troubles . . . stalling out at random HELP

I have a 1989 LeSabre with 265,000 miles -- it's probably one of the more reliable cars I've owned and a couple of years ago I drove from New England to Southern California by way of Cananda without any trouble whatsoever. The past six months or so it's been stalling out for no reason I can figure -- the engine just dies completely forcing me to coast to the side of the road. It almost always starts right up again if I wait five or ten minutes. The battery is fine, the alternator is recent, starter is okay.

Last November I spent almost $1000 total at two different repair shops fixing the crank pulley (supposedly the flanges were bent, causing it to send mixed messages to the computer and making it stall, according to the mechanic who fixed it), the wiring harness (the insulation had worn out, when you tapped it the the engine died out) and god knows what else. We thought it may have been a loose timing chain, but turned out that wasn't the problem.

The two things I've narrowed it down to are 1) the coil and module unit or 2) the computer controller. Trouble is, every time I get it to the mechanic it starts up just fine, so he's unable to do a diagnostic on it. Last time I spent a hundred bucks to have it towed to the garage, since it wouldn't start even after twenty tries. As soon as it came off the tow truck, the mechanic was able to start it up immediately.

It's not just when the engine is hot -- lately it won't start up even first thing in the morning when it's been sitting all night. It's not a speed issue as it can stall at 20 mph or at 60 mph on the freeway (very scary). It can happen after two blocks of driving, other times it'll be fine for a hundred miles or more. It may be an RPM issue, as it tends to happen when revving or increasing speed -- maybe knocking something loose? Again, it always starts up again at some point as if nothing had happened.

Any suggestions would be much appreciated. My local mechanic, who is an honest guy, said it might not be worth fixing but to me it makes sense to try since I know exactly what is right on the car and what isn't -- a lot of parts have been replaced in the last couple years. If I get a replacement car I'm starting from scratch again. This is a scary problem as it occasionally dies out in the middle of the freeway, causing me to lose both my power steering and power brakes -- I've always been able to shift into neutral and coast over to the breakdown lane and bring it to a stop by wrestling with the steering and bearing down hard on the breaks. (A big argument for old-fashioned unpowered equipment, imo.) Any advice?