1980 TORO 38150-826 Need Help Chain Driven by Gear Range Disc Keeps Jumping Off

I have a 1980 Toro 38150-826. When I engage the traction drive into 1st,2nd gear the chain jumps off from the sprocket?!? The chain itself feels like there is plenty of tension, but it jumps off every time. Has any one seen this before? Any help would be greatly appreciated. Thanks