I have a Toro WH 312 8 garden tractor. Recently I started losing power to my mower deck. When I get into grass (not any higher than I've normally cut with no problem) the PTO starts slowing down to a crawl which obviously slows down the belts and then the cutter blade rotation. The engine is not struggling. The wheel speed of the tractor is not slowed. I can back up to a cut area and it slowly gains mower blade speed but then quickly loses it again. The only thing I have changed recently is to put a new battery in after the old one died. Suggestions?