Bought a nail and need help!

[fluffyhead69]

So last week I made the mistake of swapping my 5 series for a b5.5 passat tdi automatic as I needed to save some money on fuel doing longer trips to work these days.

Car ran great for 2 days then was over boosting and going into limp mode. After much messing about it turns out the previous owner messed with the actuator to try get more power out of it and moved the nut that limits the amount of boost. He moved it back but didn't lock it in place so after a few days wiggled free. Anyway... Mechanic friend of mine sorted all that out and it's running great again, but does seem to run out of puff at motorway speeds fairly quickly. Still keeps going but not as much as I expected. On top of this a few times when getting on the throttle it does nothing. Just keeps coasting for a few seconds then kicks in. It's really weird.
And finally.... The box is slipping between gears. Hoping that's just gonna need a service but read there is no oil level indicator on the box so can't tell if it needs a top up?

Any help on any of these problems would be appreciated.