A number of things (and not exclusive to this machine) might cause this.
Belt stretched/worn/glazed/loose. Pulleys worn to the point of poor grip on belt.
Pulleys loose on spindle arbor, blades loose on spindle arbor.

I'm leaning toward it being one of the first items(or a combo of them) because it's not likely that a loose pulley or blade would happen to 2 positions at the same time.

On the very front end, at the bottom, of the 316, you will see a belt adjustment gauge, if it's still there. You loosen up a locking nut and turn the threaded rod until the pointer is in the proper range.

It's hard to explain without a picture, but I can take one later today if you don't understand.

The deck will need to come out, covers off and the secondary belt issue will need to be addressed.