1991 Buick Lesabre

Car has engine miss sometimes at start-up, but when I approach 45-60 mph, it has severe miss which causes whole car to shake. It goes away when throttled heavily and sometimes goes away and sometimes keeps happening. Changed plugs and wires, had it in GM shop. They can't find problem. Engine temp doesn't seem to matter, will happen hot or cold. Doesn't matter what gear its in, have shifted out of overdrive to try and get it to quit. It sometimes even misses at an idle and then clears up. It appears to happen more often when the throttle is set at cruising speed. Ie, 55-65 mph it is the worse. No check engine lights and no code according to dealer. Good luck. I give up.