89 Camry LE V6 engine pulsating and rough idle

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i have an 89 camry LE V6 w/144,000 mi on it. when i bought it the engine pulsated and it died at idle (sitting, at stop signs etc). the radiator had been replaced and a tune up done, but the there was still stop leak in the coolant and it had two engine codes, both rich and lean fuel codes.The heater core was plugged too..We flushed the coolant, connected a vacuum hose or two, did a Bearyman's flush through the intake and the pulsating improved, the codes went away, and we get alittle heat, but when the car gets hot, (high rpms, extended idle) the pulsation comes back and occasionally so do the codes. It seems to run better in cold weather as well. there don't seem to be any other significant problems, but it does gets poor gas mileage. the exhaust is rotted, but it is behind the 02 sensor. we're kinda stumped at this point. anybody have some suggestions or ideas about how to fix her? any help would be appreciated. thanx....