Ive had experiences with 02 sensors gone bad without a check engine light. Refer to a user's manual for how to test 02 sensors, its checking for changing voltage across certain wires.

You may want to also check for a clogged catalytic converter, or clogged or leaking exhaust system.

You may also want to use a product like seafoam, to clean out big carbon deposits from your your intake/combustion system.

ALso, have you replaced your PCV valve? This seems to be a common solution for some weird problems.

Have you had your valve lash adjusted lately?

Thats all I can think of right now, hopefully it'll give you a few more ideas on things to check.