1998 Other GMC Models reset computer

1998 gmc 6.5 turbo, disconnected batt's and cleaned terminals due to alternator problem. Reconnected batt cables, had hard time staring truck, driving down the road at highway speed and all of a sudden it would go into passing gear. Come to a stop and the engine would stumble then rev up. Don't know if it's the computer trying to reset itself or there is a problem with it. No engine servce light or codes.