It has something to do with the thermostat. It says Below thermostat regulating temperature

Already burning oil from leak at valve cover.

Just had 2,600.00 worth of work done to car, had it towed in wouldn't start lost power on freeway, dropped in speed. Was told had to replace fuel pump and relay. Changed hoses, back brakes and a bushing. Had car back drove just under 250 miles, car had same symtoms of hesistanting then gaining speed again,check engine light came on took car back. Passed smog in January, now they say I need a new catalytic convertor, and they will wave labor but I have to pay for parts and an oil sensor needs to be replaced. Too the tune of another 1,700.00. I have taken my car to same dealer for repairs since 2001. Please advise. Thank you.