I have a 48" Exmark Metro HP belt driven stand-behind. I bought it used last year, I'm pretty sure its a late 90's model. It runs good, starts right up, and cuts good, but lately it has started showing this weird symptom. In first gear, it will go up hills and stuff just fine, but you go any higher than that and it just stops, and if you try to push it, it acts jerky and will only move a little bit and you have to work real hard to get it turned around. Has anyone else had this problem and knows how to fix it? I'm kinda stumped.