'00 New Beetle TDI, drops speed on highway when long incline hill from 65-70 to sometimes 50! Uh, and that’s with my husband (247#) too. Local driving, just me, going up my Southern Tier of NYS steep country hills will drop from 55 to 30. Have done diesel additives religiously since injector replacement ($$$$$). Timing belt new in '06. Recent work done was replacing a relay (awesome mechanic) and tensioner assembly & idler pulley, they went and aren’t considered part of the problem. Though what do I know . . . our super foreign car guy retired!
Common issue #1 clogged snow Screen. Type in YouTube and you will see what I mean
Common issue #2 MAF Sensor. Very common on that vintage TDI. Ps the one for a 1.8t motor is basically the same and much less money last I checked.
Common issue #3 carbon clogged intake manifold, if this is the case it needs to be replaced. Try #1 and two first