1997 VW Golf III 1.9 TDI 90HP (1Z/AHU) Very low power/slow to rev at startup

It happens every time the engine is started, regardless if it’s a hot or cold start. It’s extremely slow to accelerate when driving, even slower then a normal 1.9D. In neutral it’s slow to rev up. However, once you push to around 3500-4000RPM, something seems to give and the RPM suddenly shoots up to 5000. After that power is restored and the car behaves as normal for the rest of the trip. But the problem returns immediately when the engine is shut down and restarted.

• No check engine light
• Oil level normal
• Air filter is clean

The obvious possibilities that come to mind, such as a clogged exhaust, don’t seem to agree with the fact that once the car is working, after revving up to 5000, the problem immediately returns upon a engine restart.