Well since you had the transmission serviced, I would think that there's something wrong with your level of Automatic Transmission Fluid. Check your ATF level (transmission at normal temp; with engine idling, shift into each gear waiting a few seconds in each then come back to park, waiting a few seconds in each gear again on the way back). Then check your level of ATF just like you would for you oil (pull the dipstick, wipe it, put it back, pull it back out to check the level).
Make sure your transmission cooling lines are not leaky (the two small lines that go from your transmission to your radiator). Also check for any leak around your transmission pan, seals, etc.
Check your coolant inside the radiator; do you see any oil in it?
ATF level is where you should start, and if your level is fine, your problem will probably need to be diagnosed at a transmission shop.
Wednesday, November 12th, 2008 AT 2:40 PM