Car hesitates, stumbles and stalls when given gas, no acceleration past 35 mph. on 2002 Chevrolet Malibu

Car was purchased new back in '02, no real issues until last year when this started to happen, has 67,310 original miles on it. Car hesitates, no acceleration past 35 mph, problem is getting worse. Actron super AutoScanner gives no code/MIL set. No check engine light alert. Car does not shake or make any noises, seems fine except for the acceleration issue. Car has acted odd a few times upon starting, turned it off, restarted and car was fine. Car died once at a stop light but restarted right away. Problem began months ago with just struggling to get up hills, but now it's bogging down all the time. Here is the list of parts I've replaced to try to fix the problem with no effect. Spark plugs, spark plug wires, PVC valve, fuel filter, both O2 sensors (upstream & down), coolant temp sensor, EGR valve, upper & lower manifold gaskets (lower did have leak & cleaned inside and out, lower injector O-rings, MAP sensor, throttle body gasket (cleaned inside & out), switched out with good MAF & IAT sensors (no improvement). Checked fuel pressure - ok Checked Coils - ok Checked fuel regulator - ok All this has had NO effect on the lack of accleration. Any ideas? Any suggestions?

by in Fayetteville, AR on February 10, 2013
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3 answers 3 comments
ANSWER by , February 11, 2013
scan codes and post so we can adv.poss lean codes if so install fuel gauge and drive car and watch fuel pressure to see if it falls
COMMENT by , February 14, 2013
no codes fuel pressure idle 52 (psi) at all rpm reads 58 (psi)
ANSWER by , February 10, 2013
I understand you have checked the fuel pressure,but have you checked it under accel while driving? Also check the backpressure of the exhaust for a possible restriction.
COMMENT by , February 14, 2013
pressure at idle 52 (psi) within 52 to 59 pressure at all rpm 58 (psi) proper range under 60 (psi) used vacuum gauge to check for restricted exhaust vacuum at idle 14 (hg) shoud be 17 to 22 2000 (rpm) reads 17 (hg) released throttle reads 15 to 16 and returns to 17 (hg) quickly (its within proper range)
ANSWER by , February 10, 2013
are there any trouble codes?
COMMENT by , February 11, 2013
No codes are showing up on the scanner I have. Next step will probably be taking it to the dealer for diagnostics.