I have a 1989 Eldorado with a 4.5L and 93,000 miles. The car runs well at start up until it reaches a certain temperature then it will run rough until it is fully warmed, at which time it runs fine again. During the rough warm up it sputters, misses and is pretty miserable. The rough running period seems to coincide with a slight decreasein voltage to below 12 volts. The spark plugs are new as is the Idle Control Motor. Goodyear tried a new alternator and a new battery, checked grounds and computer codes. They can find the problem and are giving up. Any ideas?