Montana 2005 - hard (up)shifts

[nikiml]

This started on may car probably around 2 years ago, very infrequently (2-3 times a year), but recently it has escalated to happening on every trip when the engine warms up. The transmission starts up-shifting hard on every shift ( the first is the hardest). No problem lights are coming up. The problem goes away (temporarily) when the car is restarted.

How should I approach diagnosing this?

Are all the hard shift trouble codes reset on engine restart?
Are the transmission codes readable though the port under the steering wheel (sorry real newbie here) ? Or there is another port for those?

Thanks in advance.
nikiml

[roadrunner2]

Harsh shifts in theses vans can be attributed to a worn low pressure solenoid in the tranny.

I had a harsh shift fault in a former 2000 Silouette and a filter and fluid change did the trick.

This is, however not always the case.

Normally trouble codes show up to 'steer' you in the right direction.

Personally, I'd take the van in and have a trans specialist check it out.