loss of power….?

[sgmira_e30]

I have an s52 swapped e30. Last week I put an m50 head on and ten or so minutes after putting it on while on the freeway home I started feeling a loss of power.
As i would gradually put my foot down in 4th or 5th there was a car shaking rumble from what seemed like my exhaust from a back up(but really not sure from where) and the rpms wouldn't go anywhere even though my foot almost was hitting the floor. I would let off and try again and it was continuing to happen a couple times. then occasionally when doing the same the car would surge with all kinds of power.
I thought at first it could be my ecu readjusting itself but then it continued and now for the past week it has not gone away.

So today I went and had a dual 2 1/4in exhaust system build manifold back with new cats. The problem though has gotten better i still feel the same issue happening…

no idea what it could be, any input would be greatly appreciated

[Adrian_Visser]

What could it be? Lots of things. Did you torque your new head gasket properly and use new bolts?
Was the "new" head pressure checked and checked for flatness prior to install?

How does it idle?

[sgmira_e30]

I apologize, I ment m50 intake manifold. Non of the internals have changed Just the amount of air flow which is why i cant tell what the issue is...

[jrdeamicis]

Vac Leak.

[Adrian_Visser]

^^^