Boost problems on Subura 2.0t 1994?

the increase is changing at random from one bar to 0.5 bar . It increase one bar in the morning with no problems and I ve checked for leak on the inter cooler and pipe and found none.

check the connectors going from the increase gauge all the way back to where you're tapping the manifold to take the pressure reading to be sure you're to get an correct reading.

Take off the hose that goes from the turbo compressor housing to the intercooler and see if there's obvious free play in the shaft

Have you ever rebuilt the turbo ?

Still sounds like a increase leak. If not, your a/f percentage mights be off.