KAD42 strange behaviour

I have a KAD42 in my boat, i have noticed a slow drop of max speed during 5 years or so, as i test always when the boat goes into water in spring i know i have almost the same test environment. Latley the engine have started to give me some things to fix.
the engine runs very nice, starts direct, when i rev up to get in speed the engine revs up quick to app. 2600rpm, the compressor disengages, and then the rpm drops to app 2300(still with open trottle)then the compressor kicks in again push the rpm up to 2600 and then disengags but this time the Engine revs up very slow to 2700, 2800 and then the turbo kicks in and the boat rockets up to max rpm.
Any ideas?, the turbo? the compresor or any sensor for the compressor? I feel the compressor disengages to early? any ideas to fix?

Normal behaviour for an older KAD Volvo unfortunately. The common cause is that the exhaust housing is worn (corrosion) on your turbo and the vane tip to housing clearances are now to big. It means that when the kompressor kicks out, the Turbo is not making enough boost and hence the engine rpms drop until the Komprsesor kicks on again. Initially it will go back and forward until the boat picks up enough speed that the Turbo can take over with reduced boost. As the problem gets worse the boat won't get much past 2600rpm and may just sit there with the Kompressor kicking in and out.
Need to get someone to remove the exhaust elbow and check the inside of the turbo exhaust housing or remove the turbo completely for checking. The problem is 90% likely to be what I described as it is very common.
The issue then is getting a replacement Turbo exhaust housing as Volvo don't normally sell them and you need to buy a complete Turbo at the rather expensive Volvo pricing. There are places that can repair the exhaust housings because this is such a common issue, but whether you have one near you I have no idea.

Thanks! Very good to get my fears confirmed, just to order and mount a new turbo.

A quick check to be sure would be to put a boost gauge on the engine and check the boost levels, but yes a new turbo is the likely fix.