Lots of hydraulic fluid, although all suggestions are always welcome.
Seems like the only causes I can think of to cause the behavior are (in order I think most likely)
cylinder is wonky inside (some sort of matter inside or seal deforming or something) and is losing extension
cylinder rod is turning somehow
the lid itself is deforming up/back or the mounting brackets are moving
I'll be out there again tomorrow. I'll mark the rod for rotation, and will mark a dowel for exact length the cylinder rod will extend max. If it does it again (I expect it will) it should be clear which one it is.