Car won’t rev when it’s time for turbo to kick in

[Lotus-jonas-sweden]

So my 1991 elan started having a problem.
When it hits 4000 rpm it feels like the ignition is cutting. This happen 19 out of 20 times. I can sometimes slowly build up boost in 3th 5th gear. 1th and 2th gear are the worst.
Feels like idle has also become a little bouncy, it goes up and down between 900-1100 rpms
I live in Sweden and there are not many experts on this here.
Does anyone have an idea?
I have searched LEC but I can’t seem to find a solution

[HJ2]

Does your CEL come on?

[esprit888]

What boost are you seeing on the gauge at 4,000rpm when the engine cuts out? and at what throttle setting? (maybe have a passenger look at it while you are driving) Possibly stuck wastegate (see wiggle in WIKI) or overboost due to modified exhaust but 4k seems a little early.