worn (really f**ked up) valve guides will give you the same kind of symptoms.
if you want to check if its the head of pistons, try this:
hook up a hose with the same size fitting as the spark plug hole to it on one end, and a fitting for a compressor to the other.
connect it up and run the compressor and see where the air comes out of.
you'll need the piston to be at BDC all valves are closed (i think theyre closed at BDC, check your cam lobes as you spin the motor).
then sit and listen to where the air is coming out of.
if its your pistons it will probably blow out the dipstick, if its the head you will probably hear air hissing out of the head somewhere.
if you cant find one, i have that same size fitting for the spark plug hole somewhere from when i was doing my valve stem seals. let me know if you need to borrow it and we'll organise something.