I just rebuilt the engine in my 2003 Ascender, 4.2l I-6, and had a valve job done at a machine shop. Put everything back together, cranked it up. It ran for a while, and then fell dead. Finally figured out that I lost compression in #5 cylinder because the valve spring retainer had popped off. Replaced keepers on valve stem and started it up again. Everything was fine at first. Next morning hard to start. Now the same thing has happened at #6 cylinder. Does anyone know why a valve spring retainer along with the keepers would come off of a valve stem?