1998 Audi A4, 1.8 Turbo

Are you getting any engine lights? is it flashing while you're bogging down? It sounds like you did everything right but overlooked some things. Check out the coil packs. If one is bad you are going to misfire. Like the car will run ok but once you get on the turbo at all it will just feel like crap. They're like $20ea and they sell them at most parts stores. I would say get your codes read and maybe pick one up then change them one at a time to see if you find the bad one. Worst case, it'll cost you $20 and you still won't know the problem.

^ if your going to change one change them all, check if there is water on your spark plugs, maybe you have a blown head gasket, lol or maybe you just need spark plugs and coil packs. or it could be a vacuum leak, it is after all a turbo car and Air pressure is everything. Check all vacuum hoses

if there was trash in the fuel it would also bog when you give it gas. Sounds like Vacuum to me now that I think About it, CHECK ALL VACUUM HOSES AND LET ME KNOW HOW THAT GOES. Hook a obd2 scanner and get the codes, and figure it out. it isn't hard man you can do it