Did you check the block deck for straightness when you replaced the head? If it wasn't flat, that will cause a head gasket to blow. Are you loosing coolant?

Sorry I haven't checked this in awhile. Was busy with other things and forgot about it.

There's no coolant in the oil. There's no milky substance under the oil filler cap. Best I can tell the HG is fine.

(I drove the pickup a little these last few days and I now think that retarding the timing did make a subtle difference in how loud the pinging was under load. It doesn't seem to be as pronounced now.)

EB, no I haven't check the knock sensor. I'm glad you mentioned it. It never occurred to me. Of course, just because the knock sensor might be bad doesn't mean the engine has to ping, so I still am trying to figure out what changed, and why the compression has dropped.

What's the best way to test the knock sensor? BTW, there is no SES light on, but I should make attempt to pull some codes, just in case.

I can't hardly believe there's anything wrong with the EGR 'cause I cleaned that sucker really well not at all long ago. And it doesn't seem to be giving me any problems otherwise.