I would definitely try the stomp test first. If it turns up nothing you might check your coolant temp sensor, especially if it runs different when hot compared to when its cool. Also check your vacumn lines like winfred stated and you might also check your air intake boot, mass air sensor, tps.
If your not loosing water, running hot or the exhaust doesn't smell sweet like antifreeze I would think you could pretty much rule out a blown head gasket.
good luck and let us know what happens.
'94 318 113k miles, '69 chevelle, '65 fb mustang,