Air compressor acting funny, what gives?!

Okay so I bought a 60 gal. 3.7hp 155psi kobalt air compressor from lowes about 3 or so years ago. It's worked great up until recently...

The issues.
About 6 months ago it began to bog down (sound weaker) at around 90psi, for about 30 sec. And then it would return to normal. I thought nothing of it as I don't know much about air compressors and it still would work fine.

Then the issue began where it would start to screech, and crank about 1/4 to 1/8 at a time. Then spin a couple times and then some times completely just come to a hault. So I run over there and shut it off, kick it on a few times until it starts running normal again.

Now it's at a point where it's just running weak. It will start normal, then begin to bog down when it starts getting hot, then do the screeching and 1/4 turn stuff, then it will start to come in and out strength wise. Normal then bog, normal then bog, etc.... I noticed it seems to do better when cold.

I now the belt is original and I can smell that belt burning when it's doing that odd cranking thing. Belt looks pretty saggy. So I take it, I need a new belt... That may fix the weird cranking, because the belt won't be slipping and it would be gripping good. Not sure if it would fix the bogging down though...

Does anyone know what's going on with the compressor? And how is it fixed?

Thanks!