Rear Derailleur Problem

[Meiko]

Im having a problem with the rear derailleur. As I'm riding the mountain bike and pedaling everything seems to be fine. When I stop pedaling and just coast it, the derailleur lifts up close to the bike frame and pops back in place. Sometimes when it does this the chains slips into other gears. Not sure whats wrong with it or if its something I could fix with some simple adjustments. Help. Thanks.

[DaveM]

It could be that the ratchet inside the freewheel/freehub is gummed up and not spinning freely when you coast. If you lay the wheel on its side and drip some lube into the border between where the freewheel spins/stays still it may free up. But I'm guessing based on your limited description. Does the chain sort of hang down across the top when you coast also?