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Did you replace the O2 on the exhaust manifold by the radiator? If so, clean your MAF and/or look for vacuum leaks. Replace the PCV valve and short rubber hose under the exhaust manifold.

Lack of power above 3500 RPM could well be a sign of a clogged catalytic converter or a crushed exhaust pipe.

"Those who cannot remember the past are condemned to repeat it" -George Santayana