I have the Polaris 380 and the last time mine acted like that was when the backup valve (the part of the hose section that is bell-shaped and about three feet from the unit that you see water coming from)cracked. You should see water coming from that valve for about a 30 second time period every 10 minutes or so, but no longer---if the valve is cracked, it allows a constant flow of water through that makes the unit perpetually back up--which counters its normal forward motion.

The only other thing I can recommend is like Waste said, to check the drive belt, which is fairly easy to get to on a 380 model, and are available at Leslie's.

Janet