Mechanic Q: misses in cruise?

while driving along in cruise, the engine misses; esp. when I step on the accelerator to slightly increase my speed. my wife says she feels that miss, every now and again, even while we're just driving along.

Mechanic Q: While driving in cruise, the car misses; feel the hesitation??

The car is consistently missing while driving in cruise. The miss is more pronounced when I am in cruise and I accelerate. What do I need to have my mechanic check to see why it's missing? Approximately, how much will the diagnosis cost? Since, this has to do with the engine which has an extended warranty on it, should there be any charge?