The stumbling occurs above 55+mph when the transmission shifts up or down between 3rd and 4th. The transmission fluid was changed about a year ago, level is correct and fluid looks good.

Any suggestions what may cause this?

Try to find out what fluid they used in the fluid change.
These transmissions use a special fluid (ATF+4), most autoparts stores don't carry it, so shops tend to put in Dextron/Mercron instead.
This fluid will lead to total transmission failure within 10,000 to 15,000 miles. The ATF+4 fluid is about twice the price of regular ATF, so many shops will substitute even if they can get ATF+4. My mother in law had her trans rebuilt at @@mc0 and they put in regular ATF, sent her home with a one year 12,000 mile warranty. 13 months and 15,000 miles later the trans was totally ruined and had to be replaced. @@mc0 offered to do the job with a 10% discount. She took it to the Dodge dealer instead, it was cheaper than @@mc0 anyway.

Try to find out what fluid they used in the fluid change.
These transmissions use a special fluid (ATF+4), most autoparts stores don't carry it, so shops tend to put in Dextron/Mercron instead.
This fluid will lead to total transmission failure within 10,000 to 15,000 miles. The ATF+4 fluid is about twice the price of regular ATF, so many shops will substitute even if they can get ATF+4. My mother in law had her trans rebuilt at @@mc0 and they put in regular ATF, sent her home with a one year 12,000 mile warranty. 13 months and 15,000 miles later the trans was totally ruined and had to be replaced. @@mc0 offered to do the job with a 10% discount. She took it to the Dodge dealer instead, it was cheaper than @@mc0 anyway.


hell yeah, you're absolutely right, imediately change filter and fluid or your trany is ruined....happened to me, too!!!
if you see any chips in the filter you change, it's to late..sorry!