Compressor slowing/speeding driving me nuts

We've been running this Husky for about 15 years IIRC and never had any trouble with it. We keep up with the oil and the condensation, but otherwise haven't done anything to it. This summer one day we noticed it slowing down randomly while running. At first it was only sometimes, and now it becomes more and more frequent. We were advised to replace the run solenoid. This didn't work. Anything else we can do? We just don't want it to give up the ghost in the middle of use. We can always replace it, but we'd rather fix it. 

Sounds like the motor is running fine but the pump is slowing down.

Tighten / replace the belt.

We did replace the belt about 2 years ago. Maybe it slipped?

Sure sounds like it could be slippage.

Bad belt, not a great fit, leaking some sort of contaminant onto the belt, having an issue that makes the pump harder to drive?

So... not much help with diagnosis, but my ears agree with ShawnG.

V-belts wear down along the sides until they're riding in the base of the groove. They still provide some drive capability but they'll slip as the load increases (pump comes up to pressure).

Do the cheap/free thing first, try tightening it up a bit while you're waiting for the new belt to arrive.

I got about 20 years out of the cogged belt that I used on my own, custom built air compressor but when it finally wore out, it had about as many teeth left as a hillbilly family reunion.

In reference to the threads title.

Never let something drive you nuts it you are within walking distance.  . .

We thought the motor was bogging down, but now I think it was actually loading and unloading. My phone camera mostly picks up the compressor sound but there was also a lot of motor sound. 

The belt spec is 1/2" of deflection. It was at about 1" of deflection. I tightened it up and it works perfectly. Thanks my dudes

For smaller compressors like this, the unloader is usually a small valve on the pressure switch that bleeds air between the compressor pump outlet and the tank inlet check valve. This is cheap but the pump still pushes air as it is getting up to speed.  As a side note, if you are on a marginal power source like a generator, adding a manual valve to the unloader that bleeds to atmosphere is of great help.  

Larger more industrial compressors use a centrifugal unloader driven by the crank shaft to perform the same function. This has the added benefit of allowing the pump to get up to speed before the unloader disengages.   Some of these larger compressors are set up with an unloader that holds the intake valve open when the tank is at max pressure.  These are used in high air demand situations or with combustion engines, in both cases the motor runs continuously. 

I'd inspect the pressure switch contacts for wear.  We also need better pictures or the unloader system.  Have you watched the compressor pump and motor speed during the slow down?  Have you removed the power source and manually turned the compressor pump?