Old EC250 idle issue and power valve leaking...

[2ndGen]

Hey all. I have a question on a 1997 Gas Gas EC250. I'm new to 2 strokes and bought this a year and a half ago. It was super clean and well maintained. After a nooby problem with the carb it's been running fantastically since I bought.

The other day I went for a ride, maybe slightly higher elevation? Not much though. 1000' higher? It wouldn't idle. Started out just having problems idling, then got progressively worse. At the end it started feeling like it had nothing in 1st and 2nd gear up hills. I messed with the idle screw and it felt loose so I got it feeling better. It started idling better but then it was idling high unless I reved it down. Then it would idle too low. Very weird. This bike never idled amazingly well but never like this. I pulled the spark plug and it's got a lot of black deposits and looks oily. I'll replace it. Also cleaned the air filter.
Here's the thing I'm most curious about. At the same time as this idle thing started happening I noticed the power valve cover was leaking goopy oily stuff out the bottom front corner. I figure it's just oil build up from the mixed gas. Today I took the cover off and cleaned a bunch up. Is this something that should be maintenanced from time to time? And would it affect the idle and the power at lower revs climbing a hill?

I'm not up to snuff on the regular maintenance on a 2 stroke so anything else helpful would be great. I ride a few hours a month so it's not getting a ton of use but I use fresh gas with a 50:1 mix.