Missing under load sounds like an ignition issue as once cylinder pressures
rise the spark-plug gap resistance increases making it more likely for the
spark to go to ground else-where if there is a crack in an insulator or the
leads are high resistance.

The other possibility is the fuel delivery is leaning out under demand:
fuel-filter, pump performamce, injector condition.

Airleaks usually only effect idle where there admitting of air is
significant compared to the engines demand. Under acceleration this small
leak is less so.

Head-gaskets have other symptoms: overheating/loss of coolant, water weting
the plugs causing misses when the engine is cold which then clear when the
engine heats up and evaporates the moisture,..comprssion leaks into the
water-jacket causing loss of coolant as the pressure pushes the coolant out
past the radiator cap, and when allowed to get worse, water contamination of
the oil giving a frothy whitish deposit under the oil-filler cap and
eventually a coffee-coloured oil.
Inspecting the plugs can indicate which cylinders are effected by the blown
h/gasket.

Jason