1998 Lexus ES300 Question: w/ cruise on 55+ after 10 min, engine spits and sputters, no check engine light

This is an intermittent problem. Happens mostly when the cruise control is set between 55-70 mph, but has happened when held speed constant at same speed. Engine starts spitting and sputtering, gas mileage goes from 25 to 16 mpg, when flooring gas - black soot comes out the tailpipe. Car following smells heavy gas odor. Check engine light never comes on. I've replaced the spark plugs, wires, and coils, did not help. No check engine light and no codes. I drove approximately 6 hours no cruise control, did fine. Next morning set cruise control on 60 mph within 10 minutes started spitting and sputtering. Drover for 60 minutes with the problem, car sat for 6 hours (off) and started and drove 4 hours with no problem (no cruise). Oh yeah, no check engine light. When not spitting and sputtering engine run very strong with consistent 25 mpg , does not burn any oil, has 160,000+ miles . I do not understand why the sensors in the exhaust don’t pick up the unburnt gas or soot. Not sure which cylinder or whether the computer or injector is being held open??? Seems to do it more during cold weather 25-45 degrees.