Don't know what year model your Horse is, but my 1987 Horse uses only ONE drive belt. Although there are two sets of pulleys you use the belt on either the front two or the back two pulleys, depending on whether you want the high speed range, or the low speed range. For the high speed range, you use the pulley set that is closest to the engine. If you've got two belts installed, so that you're using all four of the pulleys, AND IF YOUR HORSE IS A 1980'S MODEL, take one of the belts off. (I don't know about newer models.)

Also, my troubleshooting guide says that if the unit won't stay in "forward," the belt is too tight, and the block should be moved UP slightly - so if you've got the block all the way down perhaps that's too tight.

There is supposed to be 1/4" air gap between the roller and bottoming out on the wedge. Move the block up a bit.

I had the same problem with an older Troybilt. I just went with slightly shorter belts purchased from an auto parts store and the problem was solved.