Help, my '01 Disco II engine is missing when I climb hills. Feels like bad gas but it has been doing intermittently for months.

Trouble is I am not getting any codes so it is difficult to know what to do about it.

I would think that any ignition-related misses would through a code.

I would think the cam sensor would notice virtually ANY miss and throw a code - but nothing.

My question is what could it be?
I have a new set of Magnecor wires and platinum plugs on hand but haven't put them in yet.

Could it be a fuel injector? would that throw a code?

Could it be the transmission slipping slightly giving the feel of a miss?

Could it be the Mass air flow sensor? would that throw a code?

Could it be an air leak somwhere?

Any thoughts would be helpful