motor runs fine cold, gradual power loss until eventual stall.whats wrong?

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seems to have good throttle response for first 5 to 7 minutes, then gradual loss of power through rpm range. after 8 minutes, car starts to idle rough, and a hesitation/miss develops on quick, hard acceleration, followed by engine stall. the fuel filter has been replaced, and the catalyitic converter is missing. there is no check engine light visible....