1999 Venture Hard shifting

I have read a few posts in here about Venture shifting problems... My wife drives the vehicle mostly but I have been in it when it shifts hard, doesn't seem to do it all the time...I took it to three garages today. The first and third garages were able to scan the computer for messages and told me that there was an error code that said something like "max adapt" and that pressure was building while driving at highway speeds to compensate for some sort of internal problem with the transmission. Then when traveling at slower speeds the pressure is still built up and so it causes hard shifting. Both places said that the vehicle needs a rebuild/overhaul..min $1800 according to them. The 2nd garage I went to said that it wasn't a big deal and that it was operating normally for that particular transmission. I dont know what to do. At one of the garages they told me there was another error code for an Oxygen sensor B1S1? they couldn't tell me what it was for and I haven't had the check engine light come on at all. Could this be the code for Mass Airflow sensor? Anyway, any ideas if I am facing a serious problem or a general anoyance would be appreciated.

Thanks Ryan

trying pausing bout 2 sec before you hit the gas when you shift from park or neutral to drive.

Mine did the same thing, but also threw a mass airflow sensor code. I replaced the mass airflow sensor and it has shifted fine ever since. I'm not saying it will fix it in your case, however, just something to consider. I've also hear there is some sort of solenoid in the transmission which causes it to shift hard, its like a $50 part, but labor is costly.