what should i check?
eng just rebuilt, has cool fuel system
new reg and fuel pump new spark plugs
oil pressure drops to below 20 pounds when or before eng cut outs.
rpm's jump all around when happens.
any ideas?

Kinda sounds electrical.

Being that the tach is jumping all around,
whats the voltmeter doing when this happens ?

If that's ok, and you really only have 20 psi of oil pressure
at 4000 rpm's, you better get back into the engine and see
where the pressure loss in coming from.

jt

It does sound electrical to me as well. Check all of your grounds.
I also had a similar problem with the ignition circuit on my boat a few years back.
My engine would do the same thing, cut out at high rpm. I ended up changing the ignition switch and running a new purple wire back to the engine. It could also be a bad tach or coil...
My money would be on a bad wire some where.
Good Luck!

What kind of distributor are you running? Last fall I had a problem with a MSD distributor (model#8360). It would run fine when cold, but after warming up, it would stumble over 4000 rpm's and sometimes cut out but work fine below 4000. It turned out that the circuit board was bad in the distributor.

Try disconnecting the tach.

Check the coil as well. I've seen that happen before.

it was two ideas one new oil pressure sender and a vac hose was disconected