"So I tried a few of my old tapes to see how they sounded. Some of these tapes were 1800 ft and I noticed that they would play for a coupe of songs and then start slowing down."

That is not a symptom of a bad pinch roller. I believe you might find heating up motor bearings, or capstan bearings, or maybe even a leaky motor cap. When pinch rollers go bad they usually do not wear or harden exactly uniformly and they screw up the tape path. It will hit the guides and actually even get damaged. This was VCRs, and the tape path in those is every bit as critical as audiophile or pro R2Rs if not moreso.

I would run it as far apart as possible and see if the RPMs go down playing, then see if they jump back up not. And then if they do, try to slow it down with your hand, maybe with a rag. I would do the same to the capstan.

I doubt you are mistaking muffled sound for slowing down. A pinch roller CAN cause that by affecting the tape path.

Ho about recording a test tape with like a 1 kHz signal on it. In fact make it a square wave. You can read the timing of it, and it will reveal the response curve. I say that because this whole thing doesn't sound right. There is an argument about pinch rollers now but that matters not. The load on the motor from ANY pinch roller (you kno you CAN use metal if you cna machine it well enough) should never pul down the motor RPMs. In fact even running Scotchtape (I mean the kind you stick to things) in it, it should break the tape, not stall or slow down. It should not happen. It should take a pair of Visegrips (tm) to stop that thing.