so, they're clean, that doesn't mean they aren't weak and opening at a lower psi or the seats are worn and they drip. if they were replaced a while ago and
cleaned recently, i'll let you off the hook. if those are the original 1s, replace them, they are worn out. bosch told me a long time ago, i don't care whose car-bmw, mb, saab or volvo-or how often they are cleaned, they are still metal parts and metal wears down. i've done it to all my cars and quite a few others.

what else, how old are the plugs? what are they? wires, cap, rotor? maybe the compression is low on 1 cylinder? check the valve adjustment? is the co set right? does it have egr? or an o2 sensor? 80s came either way. lotsa ways to go. good luck, chuck.