There is a lot of things, low compression is another thing to consider. It may almost fit your problem too, like a blown head gasket or valve adjustment.
Not easy but removing injectors and checking spray pattern would be on my list. Adjusting valve lash is another. Im grasping at straws here, but my preference is start with the cheap/inexpensive stuff first.
Compression testers arent all that expensive, but expensive enough to make it hard to justify for a one shot deal. Perhaps renting one or maybe an aquaintaince has one he could do the test for you.