1996 Other Oldsmobile Models stalls at stop sign/light

96 olds 88.3.8l.once in a while it will stall when at a stopped position when in drive.it will drop to a low idle then surge then stall also when i stop at a store when i come back out to start it sometimes it will not start,it will turn over but not start.sometimes 5 min or 45 min of turning ot over it will start and be ok for a while.i get two codes,po336 crankshaft reference signal circuit and the other code is po135 upstream heated 02 sensor-heatercircuit fault(bank 1, sensor 1),thanks