Loss of power

[RonPellerin at aol.com]

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My 1988 90Q 5cyl 10 V seems to loose power at around 3500 RPM when
accelerating. It reminds me of a tight valve clearance or restricted exhaust,
except, once you go WOT it accelerates fine (for a 5 cyl 10 V). I have heard
that a defective knock sensor, O2 sensor or catalytic converter could do
this. When I cruise at about 65 mph, constant throttle angle, I get the check
engine light. If I back-off then give it the gas the light goes out for 5
min, then comes back on. Anybody ever experience this or any suggestions?
TIA,  Ron Pellerin