The polaris 280 spins in circles and the rear end is standing up so all wheels are not in contact with the floor of the pool

I had recently done maintenance on the Polaris, and when replacing the wheels had put the back wheel on upside down. The axle is off center on the mounting block, so upside down means the back wheel is a little farther away from the idler wheel than it should be. The effect is that the wheel touches the idler wheel, and looks fine, but when in the water and under pressure it loses contact with the idler wheel, so you don't get power from the back wheel. I found this by putting marks on each of the wheels to count rotations while the Polaris was moving around the pool. It was then easy to see that the back wheel was slipping.

Try moving the float closest to backup valve slightly. This can help it stay flat on bottom. Mine was riding on its back wheel and kept climbing too high and getting tail stuck in skimmer...and now it is riding smooth across pool floor now.

there are several things that may cause the unit to run in circles. #1 never roll the hose when the unit is out of the water. Infact every year you should remove the entire unit from the pool and lay the hoses straight on the deck in the warm sun light. 1 to 3 days is usually enough use rocks if you need to in order to force th hose to stay straight. the hoses have memory and when you roll them up and lay them outside the pool this starts the problem. you also need to see if all three wheels turn in sequence. you can grab ahold of the single wheel on the left side and be sure it spins forward and backward without resistance. additonally the right front wheel should spin at the same time. there is a gear that runs between both front wheels and when the booster pump runs the gears spin and turn the front wheels. the back wheel turns from the tension placed on it by the small wheel between the two tires on the right side. over time the metal gears can wear out the plastic grooves inside the large wheels. if there is too much wear on one of the wheels you can remove one of the front tires and swap it with the back tire since it does not need the gear or grooves to turn. another issue is the directional jet located under the fitting the whip hose attaches. this uses the water pressure to direct the unit in different directions if the nipple is pointed to 12 noon this causes the unit to climb more 6 am will limit the climbing the urge. when pointed to the left it cause the unit to vere to the right and vice verse. 808-871-POOL

Make sure that your thrust jets are properly positioned. You will also want to check for any obstructions in the wheels or axles that would restrict movement. Often with these automatic pool cleaners, one of these two problems is causing restricted movement in the wheels.

It sounds like there might be water in the head of your automatic pool cleaner. Take the head off and shake it. If you hear water sloshing around, you will need to replace the head.

When my polaris wheels stopped spinning it was the water jet which spins the turbine was clogged. The in-line filter had failed and some small debris had passed through to clog the jet. Easy fix to take off the top cover then take off the turbine cover and examine /clear the hose (clamped on by zip ties) and the plastic jet.

Back wheel does not drive. What can it be?

If the cleaner is going too fast then it is likely that there is too much water flow at the drive turbine. Is there a pressure relief valve in the water supply line (at the edge of the pool)? If so then change the tension on the spring to allow less water to get to the cleaner's drive turbine.
The speed of the Polaris is probably the cause of the front wheel lifting as well, however if (after adjusting the water pressure) the front wheel still won't stay on the pool bottom, then perhaps adjust the little jet at the back of the casing. Mine points a little "down" towards the pool floor and this tends to raise the bac k wheel (or alternatively keeps the front wheel in contact with the pool bottom or wall).

By the way, if you want to see how the cleaner works, then search for eHow and lookm at the repair/overhaul of the Polaris 180............. the mechanism is similar.
Good luck.