Boosting Issues

[blairtosh]

Hello
Let me start by saying i have searched this topic for hours and hours and cant seem to find a solution that works for me.

Engine spec:
1990 redtop sr20det
full 3" straight pipe exhaust
greddy IC and piping

I am having issues holding boost while on track, been like this for way to long now and I cant seem to find the issue.
what happens? - Basically at ~5000rpm my boost drops off all the way and my gauge reads 20VAC, I can maintain the ~5000rpm but it has no boost. Originally i thought this might of been a fueling or ignition issue but I cant figure it out. If i put the clutch in and let the revs go down to idle then re-clutch kick there is a 50/50 chance i get boost back. I was using a blitz dual boost controller and the lower the boost levels the less frequently it happens. I have since removed it and tried stock wastegate pressure with no luck.

Things i have tried to fix the issue:
new fuel pump
blown out all fuel lines and made sure they are free of blockage
new fuel filters
tried new injectors
new spark plugs
different coil packs
new air filter
all new turbo side gaskets (turbo looked in good condition when i inspected it)
full boost leak test, most recent one being yesterday
removed BOV in-case it was opening prematurely
all new vacuum lines

I have not tried my MAF but my understanding is that it will have no effect on boosting pressure since there are no electronic valves involved.

Any help is appreciated
Thanks

[jr_ss]

Try testing your wastegate. They do tend to fail with age. Check turbo gaskets and inspect for wheel damage and freeplay. You aren't going to make boost if the damn thing doesn't spin.

[TwoForDeh]

My suggestion is what jr_ss said. I would test your wastegate.

[derass]

Are you still on the throttle when it goes from full boost to -20 in/hg? That doesn't sound right, if you lost boost with the throttle open, you would show 0. There might be something wrong with your gauge. Is there a noticeable loss in power?