Thanks so much for your help. I'd gone through these procedures with no luck.
Finally, I called HP tech support, paid $99.99 for a year of support, went through extensive diagnostics with the tech and learned that the mother board is going bad.
HP wants almost $300 to replace the mother board, in addition to the $99.99 I just paid them for tech support. That's more than I paid for the computer brand new at Best Buy.
First, the motherboard should be the last thing to go. I've never had a motherboard go bad. Hard Drives? Yes. Monitors? Yes. But never a motherboard.
Second, it takes less than 30 minutes to replace a motherboard, and the board likely costs HP no more than $50 to $75.
I'm inclined to simply find a used Pavilion g4 on Craig's List and swap boards myself... and then bid my farewell to HP in favor of a Mac notebook.
Again, thanks for your help.
John