Fuel pressure may be dropping. Connect a fuel pressure gauge to your fuel rail or fuel distributor and take the car for a romp. Your fuel pressure should rise slightly with full throttle. If it drops or withers away you have a restriction in your fuel supply. A blocked fuel filter, fuel pump may be tired, etc.
I find that it's easy to see while I'm test driving if I tape the gauge to the windshield.
Certifiable VW Mechanic
20 years VW/Audi Repair, Maintenence, Restoration