If you have Silverleaf, check for error codes. Also, it sounds like you are seeing this more frequently now, so perhaps another shop could check for codes for you.
If it is a Cummins, go to a Cummins/Allison certified shop. If Cat, then a Cat shop.
Sounds like an intermittent problem between the trans and engine computers, where it is hitting both the trans (to 3rd), and now the engine, loss of power.
Not much help, and probably what you had already figured out
Might also check in with the CC Yahoo User Group.
Best of luck,