On the 4.9L Cadillacs & 1990 4.5L's, you need to follow the negative battery cable where it mounts to the side on the engine block (next to the starter). You need to take off the negative cable & clean not only the terminal lug, but the stud that bolts into the engine block. Remove any star washers you find on the lug, they are part of the problem.
The cause of the misfire is that a loose or dirty connection at the ground wire will cause a voltage drop on the distributor circuits. As little as 1/2 volt drop at the distributor, will cause the computer to lose the distributor signal & cause the engine to misfire... any electrical load (power windows, blinkers, etc) will increase the voltage drop at the distributor circuits & cause the computer to lose it's sync with the distributor. An easy test to start the engine cold, then rise & lower ALL your windows at the same time. If the running of the engine is affected, clean he main ground on the engine block. We call it a "Cadillac ground service"