Well, you've gotten more than your money's worth out of that box! I'm no expert, but with that mileage, and the symptoms you mention, it would seem to be highly likely that you're due for a rebuild, even with an excellent service history.
Mine went out at about 140k miles with similar symptoms. I had mine rebuilt by a local independant mechanic who had worked and trained at the local dealership, so I knew he'd do a good job. Cost about $2000.
The other alternative would be to buy an MB factory-rebuilt box. Comes with a nice warranty, and includes all the proper updates to fix prior glitches, like the B2 piston failure. Costs a bit more to go that route.
Just for giggles, have you checked your fluid level? Could be low (but you'd still be living on borrowed time).