My Prowler 920 has worked fine for 2 years and now seems to get stuck climbing walls. It runs OK on the bottom and out of the pool (upside down) but on the walls it gets halfway up and gets stuck. Once it is halfway up it tries to go forward for about 2 seconds then stops, tries to go forward for 2 seconds then stops, repeat. This action seems to have the effect of not getting it to the top of the wall. It also never realizes that it is stuck and goes into reverse to get out of there. Just stays on the wall until the cycle ends or I kill it. Impeller seems clean and I changed the tracks to new ones. Any other ideas?