Actually I don't like it when it happens. It could recur at any time.
if it does I would look to the mode selection, which is what switches the IC's gain and who knows what else for the on screen display. While it doesn't seem logical the sweep would be compressed if it was stuck in that mode, the input could have been out of range.
I could be off base here, this is my first post and I have to admit to not reading the whole thread.
But I got my feet wet. Soon I will be ready to start a thread maybe the old timers might like - a 561. Has troubles in the vertical section after an arcing transformer threw the DC from the negative bias into the vertical. I have much information to put together so it probably won't be today. Honestly, some of it will come from my posts on Usenet.
Be well all. See ya.
Jeff Urban