Misfire cause?

My head gasket went on Sunday it boiled the header tank, gauge never went over half way though? Refilled the tank at each stage, it did it twice on the short journey home (about 6 miles). Pulled it apart yesterday, checked everything, cleaned everything and put it all back together. Starts and runs perfectly, doesn't smoke, no bubbles in header tank and heater working as normal.
Now, when it gets over 3k rpm under load, there is a slight misfire, accompanied by a small puff of blueish tinged smoke and a loss of power. Before she used to happily hit 3.5k rpm (tuned pump, all 3 stages done with full width allisport cooler and silicone hoses).
I'm thinking fuel related, possibly filter or lift pump?
What do you guys think?

Id say lift pump or injectors need cleaning or replacing.

Blue smoke is oil.. How far have you driven since gasket to try clear it? Did you get a skin and pressure test? Of not did you flat bar it to check for warp. Did it get
Hot and possibly crack a ring.... Lots of reasons this could be happening after a big job like head. But because you say blue. I'd say its oil. But it may not be blue. Then it's steam....warped head. cracked head. Not clean enough...

It looks blue in the mirror, but could just be the tint on the mirror. Only driven about 2 miles since rebuild. Head was checked and perfectly square.
Ran it upto temp on drive first to make sure all was well before going out. Few bubbles while ticking over, but none since (put it down to air circulating out of cooling system - did fully bleed it first before starting) no excess pressure in header tank and no drop in coolant level.
Will see how it goes today