Sounds like you have an exhaust brake and it is doing what it is supposed to - set a target gear (look at the shift panel - usually 2nd or 4th gear) and start down shifting to help you brake, when you take your foot off the accelerator pedal. If you press the accelerator pedal, does it go back to showing 6th gear and act normal?
You should have a switch on the dash, maybe marked Exhaust Brake? Turn it off and see if that cures what you're seeing.
Rick and Larrie Dee
1997 40' Newmar London Aire DP CTA 8.3 (Mechanical) 325 Spartan MM
Bringing her back to her glory.
'08 Jeep GC Overland.