67 Barracuda rallye speedometer issue

So this is technically not an Electrical and Ignition issue, but I didn't see another category that was any more appropriate. I got some good feedback in this section on another gauge-related issue, so here goes. I also recognize by the way that this request for help may be premature, as I will not be able to pull the cluster again for a while, but I'm hoping perhaps someone may have had a similar issue and can offer advice just based on a description of the problem.

So a week or so back, I removed the Rallye instrument cluster from my Fastback to replace the vacuum gauge with an aftermarket tach and upgrade the lighting to LEDs. Since re-installing the cluster I have had an avalanche of problems (related thread concerning faulty OE gauges and the tach). I was careful about re-connecting the speedo cable (thread on connector), and everything seemed fine at first. I did happen to notice after driving it a bit that the needle on the speedo would occasionally modulate a bit at a constant speed, which was something I've not seen before. It was minimal and I thought no big deal.

When pulling into the parking lot at work yesterday, I heard a god-awful clunking noise and had no idea what it was at the time. On the way home, I noticed almost immediately that the 120MPH speedo was acting weird and making a clicking sound. It would read no lower than 10MPH at a stop and no higher than 30-40MPH when moving. I pulled over at the earliest opportunity to check the cable connection. Cable was solidly mated to the rear of the gauge and the odometer was and is still spinning freely. I could also spin the speedo in both directions by hand with the cable disconnected, and got almost no response from the needle. It seemed to turn freely without binding and not make any noise.

Sounds to me like the speedo has or is coming apart internally. As I mentioned, it will be a while before I'm able to pull the cluster again and have a look, and I do need to drive it at least one more time in a couple of days. Should I disconnect the cable just in case? Don't want to mess with odometer accuracy, but also don't want to risk damaging the speedo further if it can be saved. No idea if these can be rebuilt, or if I should just start looking for a replacement. I'm sure they are cheap and readily available lol.

Any advice or tips from someone who may have had a similar issue would be most appreciated. Thanks!