Boost issues

I have a 2001 jetta with the 1.9 ALH TDI.
It seems like it's having issues it builds boost great when you first start, but once it's been running a while on the highway in 5th gear it seems to fall on it's face. I just got the car and am new to the vw world but I know it had a 3 in turbo back exhaust. Any advice or help would be appreciated.

Is there a check engine light lit?

There was It was throwing 2 codes one was for the glow plugs which I replaced and it went out the other code was p1557 but it's not showing any now at all

You probably have sticking turbo vanes or VNT linkage, or possibly a bad N75, or a restricted vent line for the VNT.

That DTC refers to an OVERboost condition. If the ECU detects too much boost, more than it expects to have under its control, it puts the engine in limp mode.

I appreciate that I'm actually pullin the turbo off this weekend to do seals in it. So I think I'll clean it all well and put it back in and see if that changes anything and then clean the egr valve untill I can get the money up for the delete