It could be two things...

1) climate control

2) blower motor

but before you go nuts and start replacing parts, look for loose connections. There should be a 2 pin plug on your motor, see if it's loose. There is also a wind force sensor on the fan housing with a 4 pin plug on it, the blower motor will not work without this.

I've rarely seen a blower motor go bad because it's a simple device; your problem is intermittent, which makes me think that you have a loose wire or the climate control is your problem.

-jonathan