Question regarding Sole treadmill

The treadmill has ~100 hours and 600 miles logged

From what I've been told by a few service people that work on all major brands of treadmills, that is the sign the deck is going. That is usually from humidity. The decks on treadmills are made out of high density chip board (higher than MDF) which is then coated in a variety of anti-friction substrates; one being Teflon. The glues that hold it together tend to get weak and the structure deteriorates and there is more flop in the deck. Combine that with a belt that has been under tension for a while in one place and you get a factor of heat from slippage.

Chances are the deck is shot from being in storage with humidity and the belt stretch is just from it sitting.

Good thing, most decks are under $200 and a new walking belt can be had for under $100.

That is why you should never put a treadmill in the basement or an area where there is excessive humidity. Along with excessive heat.

I try to use my treadmill once a week even if it's just for a cool down or put a couple extra miles on while watching the tubeset (TV)...

If it is getting g hot, that is a sign of friction. It either needs more tension or more lube. Since it is slipping a little, I would start with tightening it. If you are worried about 1/4 turn, try 1/8 of a turn and then test.