Lots of good advice here, but I gotta ask............. In ten responses nobody has asked some preliminary questions.

Did this problem just start happening? Has it been going on for a while? Why did he replace the power valve? Was that his first attempt to solve the "problem"? If it just started, what else was done to the engine before it started, even if stud doesn't think it's related? Has he done a compression check? Maybe some of those answers will help the helpful and knowledgeble here zero in instead of "shotgunning".