Speedometer trouble

Hello all, I'm new to the v6 part of the forum. I have a friend that has a 98 3.8 that I changed the transmission oil/filter on it some time ago. After it was done, I took the car out for a drive and the speedo went all the way around and came up on the other side of the stop. I noticed the needle was wanting to move, so I opened the gauge cluster and reset the needle. The cluster worked well for quite some time after that, but it went up and over again and has just been stuck there. What is causing this? A faulty speedo gear/sensor? Is the gauge cluster bad? (did I break something?) Any insight is greatly appreciated!

Thanks in advance,
James

The stop is not missing or anything like that. It was working normally and when I reached 55mph and started slowing down, the needle kept climbing and went all the way around and decided to just hang out there.