Checked everything: compression, valve adjustment, ignition, fuel everything works.

When I start, vacuum is good then about 20 seconds later It go's crazy shakey and low. Stop let sit 2 min's start again and same thing.

What is it?

Carb or EFI, crossed plug wire.

250 blower with 2 holley 850.

please explain crossed!

2 cylinders firing out of order, spark plug wires are on wrong cylinders, still idles, vacuum dances all over the place, power is down pulling out. just a guess.

If a plug wire was crossed the bad idle would consistent.

My idle starts good and then go's bad in 20 sconds. Would a crossed plug wire do that?

Is this a new build? Maybe a lifter (or several) adjusted too tight. When they pump up, the valves are hanging open. Or broken valve springs

Could be a leak. Spray brake kleen (non chlorinated ) around all vac ports (like the boost port for gauge) and see if revs jump up. Also try around base of blower and intake....

No probably not, any black smoke when it's doing this ?

Does the engine sound like its missing at idle and shaking? Or is it just revving up and down like blower surge?

Restricted exhaust?