Help my 1997 SC400 has started running bad and stalling after coolant temperature is fully warmed up. Car runs fine when first started and drives good but starts bucking when driving and surges and stalls at stop. Check engine light is not on and no codes are pulled. I have done 2 things recently - filled engine fully with coolant as heater did not work and changed valve cover gaskets as oil was leaking. All lines and wires are attached. Would appreciate ideas. Thanks