The engine had an obvious miss at idle earlier, which would come and go at higher engine speeds, and yesterday I changed out the spark plugs (ACDelco iridiums) and wires (ACDelco Premium silicones). Whilst I was at it, I did some other maintenance as well - Genuine GM belt, ACDelco fuel filter, ACDelco Supreme DOT 3 brake fluid, transmission drain and refill with a Genuine GM filter and ACDelco Dexron VI. Also dumped in a bottle of Lubrizol combustion chamber/fuel injector cleaner. Afterwards, the brakes were MUCH better, transmission shifted MUCH smoother and the miss at idle was was less noticeable. There is some engine vibration at idle (mounts are good), which disappears at higher engine speeds and it drove like a dream. Fast forward to today, and the engine vibration is very noticeable at all RPMs. I was thinking about changing out the distributor cap and rotor, and thought of doing a compression test, in case it's a valve (although I somehow doubt that). Any ideas on what else could be the culprit?