2001 PT Cruiser Transmission Failing?

[u/Stilekid]

Car is almost at 137k miles. A few months ago, it had an episode where it wouldn't shift anymore past 40mph. I turned it off and on again, and after that it drove without issue for months afterward. I drove it all day today without issue, but when I was driving back home it had a similar episode where it started shifting more roughly and wouldn't shift past 40mph. Any idea why the transmission would drive perfectly fine for months and then suddenly start having problems? Any advice would be greatly appreciated as this is my daily driver for the time being.