[Davescort97]

The pump which supplies pressure to the clutch packs is probably getting low on pressure. This could be from low fluid level to dirty screen. If you cycle the shifter into all drive modes before you start out, it will build up pressure and you will be all right as you pull out. Since you just put in new fluid I would rule out the low fluid level and dirty screen. Sometimes in trannys that have high mileage it's the muck in the fluid that keeps them going. You change the fluid and without the muck they will not shift properly.