I'd first check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.
If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues
Also check that the vacuum hose running to the dump /or turbo bypass valve is intact with no leaks as if the dump valve is opening prematurely you'll be losing all your boost
The dump valve is on the turbo boost air (post compressor) side its there to blow off or 'dump' the excess boost pressure you get when you close the throttle ( like just before a gear change)
the important thing with a dump valve is that it seals properly when its not being operated if the valve has a torn seal or a broken spring then it'll blow off before it should and you'll lose boost pressure
this video shows how you can test it
one other possibility is that the wastegate actuator spring has gone soft so allowing the wastegate to open prematurely - if your actuator has a threaded end you can increase the preload on it to see if this improves things otherwise these units are quite cheap compared to a new turbo so it may be worth trying a replacement.
as for the wastegate then this is on the exhaust side and 1st thing to check is if the wastegate 'penny' or flap can be fully seated
if you remove the wastegate actuator and then look inside the turbo exit you'll see the wastegate penny moving back and forth as you move its arm if you smear a little grease or engineers blue onto the face of the penny and snap it shut and reopen it you shoudl hopefully see the seal line all around the penny
if thats OK you can then test the wastegate actuator with compressed air to check its opening up this shows if its going to be able to control boost
this video shows how ( from about 2 mins in)
and to check the spring ( is it blowing off too early) you need a spring balance or some other method of measuring the spring force as shown in the previous video