this post is sorta long so I will begin with the problem and then provide some of the test results. I failed the CA nox a few weeks ago. I replaced the O2 sensor and adjusted the valves (change 4 shims - about .004 inch variation). The car started right up but within 1 minute it began to stumble (idle speed began to bounce between 900 and 300). If left idling - it would eventually stall out (in 3-5 minutes). If I tried to drive the car - it had no accelaration or power, I have to continuously pump the accelerator to keep the car going (or the engine dies). Test results- It is not a plugged exhaust system - I unbolted the flange just before the cat converter and let the exhaust free flow - no change in behavior. Disconnected the o2 sensor- no change in behaviour. No codes are set. the fuel pressure is 43 to 45 psi. A compression test shows 195-200 psi for all cylindars. the spark plugs were replaced and the cap and roter were cleaned (they are about 10,000 mi old). The head was redone about 80,000 mi ago. (the car has 160,000 mi now. I am going to do a cylindar leak off test later. Could bad valves or weak valve springs cause this problem?? Any other ideas??