Check the PMV reservior. Is it getting fluid from the master cylinder? You should see fluid in there. It may be low (but the sensor should catch that?) and might be trying to pressurize the lines. It builds pressure up behind isolation valves for traction control purposes (not ABS), then releases the pressure in spurts to the brake cylinders to control traction.....applied braking. If all seems well here, and your traction control and ABS are working correctly, the pump itself may be going out. This will be a PMV replacent. The pump isn't serviceable by itself. You're talking about 300 bucks, and a messy dirty nasty job. I've done it twice. Don't ask. The good news is that you'll almost entirely flush your brake system by the time you're done bleeding.
Now if this all sounds like doom and gloom, can I ask if you have the Electronic Level control? When you accellerate, the back of the car settles......see where I'm going with this?