Stalls

Hi. I have a 1999 Ford Ranger XLT with a 3 liter engine and 60K miles.
The truck was not shifting between 1st and 2nd so we took it to the dealer.
They changed a sensor and fixed that problem.
While it was in the shop, they performed a 60K service and also changed the serpentine belt.
They also said it needed an O2 sensor (very expensive). We declined that for now and started running 89 octane as suggested by the manual. The check engine light for the O2 sensor has stayed off since.
However, now once a week or more the trucks' check engine light will flicker rapidly and the engine will die when decelerating. It usually will restart right away or with a little wait. We never know when it will happen.
Any suggestions appreciated. I noticed in the Q/A section that a similar problem on a Dodge was fixed with an idle speed control motor. Could that be it?
Thanks!