I would check and do a few things without being able to see the car. 1st of all as xsperf said jack it up and take the wheels off and take a look, you might need pads/rotors and a brake bleeding. If you have not bleed the brakes in a while and the pads/rotors look alright I would do that, it almost sounds like you might have some air or moisture in the lines. One other thing you could try is pulling the ABS fuse and seeing if it happens then, if not then you have some issue with the abs. I would still suggest changing the brake fluid, it should be changed every year.
Here is a walkthrough for the front pad changing that I did:
Brake Pad Change
Here is another one by Amanda:
here is a brake bleeding procedure also by Amanda:
If any of that doesn't solve your problem it might be time for a new brake booster.