I would change the engien air filter in case it hasn't been changed for a long time. Next, I would look at the catalytic converter. A partialy clogged cat can cause that symptoms.
Also can you elaborate more on the issue?
Do you notice it at certain speed? Check engine light ON?
I would like to measure the compression ratio per cylinder and see the differences, it may be leaking, and it should determine if the engine is within the compression ratio compression standards of 9.5 to 1