Toyota Repair: Pickup idle problem, air flow sensor, intake tubes

Question
I have a 91' 4x4 pickup and its had a problem for about the last 2 yrs that I just live with.  When idling, the engine will "shudder," the idle will rise a bit, and then quickly drop down to normal, and repeat this at a steady frequency.  The chance of it doing this is increased if I have my foot on the brake; although, it will do it sometimes even if the brake isn't depressed.  That makes me think it is a vaccuum problem, but I also think it may be an air flow sensor in the air intake, but I'm not sure.  What list of things can I check to pin this down?  Thanks.

Answer
Yes it does sound like an air leak of some sort, have you checked for any leaks on the intake tubes from the air filter to the throttle body, there are other possibilities including an EGR problem, if there was a problem with any of the sensors the check engine light would turn on, by the way is this a 4 cyl or V-6 engine?