I will investigate the plug wire ends. Although, cylinder number 4 had the problems with detonation? and that is the one that had an injector seal air leak recently (replaced since then). Coincidence?
The valve guide seals were replaced about 20k miles ago and, at the time, I was told that the valve guides were loose and I would have to replace them in the near future. I also have the "typical" front cover seal leak. My car has now over 240K and it is consuming about a quart of oil every two tanks of gas.
I think it is time...