Engine misfire -- 98V70AWD

[MK]

I'm stumped--- I cannot figure out why # 3 cylinder is misfiring. I replaced plugs, wires, cap and rotor. Ran fine. Check engine light comes on a month later-- cylinder # 3 misfire. I checked plug-- slightly damp and black. Other plugs nice light color. Checked plug for spark--very good. Swapped plugs around, same result. Unhooked fuel injector wire connector on #3 , no difference in the rough runnning. Unhooked other injector wires one at a time---noticeable difference-- . Took injector rail out, Marked injectors, sprayed cleaner through them, new o-rings, re-installed them, swapping the #3 injector with the # 4 injector. Still rough running. Un-hooked #3 wire, no difference. Unhook others, big difference. I checked the plug for spark on #3 and it is good. Compression seems fine. Onlyn thing I can think of is a wiring problem to the Injector--How do I test this? I had a motor installed in this about 10000 miles ago--it had 92000 on the replacement. 98V70AWD169000mi[/list]