Riding Lawn Mower has no power moving forward in higher gears.. weird, BUT could it be that...

My mower is four seasons old. A Poulan 17.5 HP, 42, " 6 speed, model PO 17542LT Last year I began to notice that the mower would delay taking off in the higher gears. I had to start in the lower gears and then shift into the higher gears after the mower started moving.
Now the problem is getting worse and the mower will hardly move at all going up an incline. The engine sounds normal. Doesn't make any change in sound when under load, sitting still or going at a good speed on level ground. Oil, all filters changed, and clean.
I was wondering if the problem could be the motion drive belt. It is supposed to be changed after about 100 hours of use, and that is about right for this mower. But to change the belt, I have to remove the "mower" part to get access to the belt.
Sooooo! Is the symptom of the problem that I am describing relevant to the belt that I mentioned, OR could the symptom be something else wrong. I would like to fix the problem without having to pull of the entire mower
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Like you say its likley to be the belt or its tension. If it still drives all be it slowly i wouldnt say the transmission or the engine is at fault. Sort of like a slipping clutch?
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