2000 GMC Sierra Long starts, attempts to stall @ lights in

Sometimes sluggish, tries to stall at traffic lights sometimes, also on warm starts has long crank times til running. Was wondering if this vehicle is prone to needing torque convertors?
Already Tried:
No check engine light present, codes were still checked and nothing has returned. Mechanic said the ecm may need to be reflashed by the dealer. I mentioned the torque convertor because the was a weird plastic gear noise coming from that area not long ago, it didn't last long and has not returned, but the other issues developed since, the long starts, rough idle at lights in gear, and tries to stall in gear, but when shifted to neutral, it clears up. Only has 58K miles on it.