I have a 2004 500i automatic that is lunging forward and squealing like either something is slipping or not in gear all the way. It doesn't do it all the time but seem to do it more when going from low range to high after a hard pull. Also it doesn't seem to be down shifting itself when going uphill, when I get to the top the engine is really lagging and almost out of power. It was recently in the shop and they replaced the pulleys and belt which stopped the squealing the transmission had but it is still lunging forward and not down shifting. Anyone else had this problem or have any ideas?

 

Sounds like the secondary clutch spring is weak. Is it also kind of boggy on acceleration? An aftermarket spring with more force is around $40.