treadmill belt jerks?

I have a Proform 560 treadmill and it has been operating since 2007. We have a 5 year warranty until 2012 and so far they have been helpful in fixing the jerking problem, until now. They came in twice, the first time, he changed the front roller and the drive belt, second time, he adjusted the tension. The amusing thing is, every time he came to “fix” it, he made it worse. I am 275 lbs and when I run at 3 incline and 4.5 MPH, It wouldn’t slip at all, but now, prior to the repair guy “fixing” it, the same settings have become unrunnable. I am not the only one, my brothers, 145, 155, and 175 correspondingly, also run on it on higher speeds, and it slips for them too. It is not a weight issue. What should I do?? contact Sears again?? or fix it myself?? If I tighten it too much like I did 2 years ago, the back roller will brake and they came in and place a new one, and that fixed it. So should I do the same thing again?? Ever since we bought it, we replaced the belt once, the front roller and drive belt twice now, and the back roller once. So should I tell them to place in a new belt and back-roller? Any advice will be very helpful and appreciated.