First off, I'd be lubricating more often than that.
In particular, I lube after EVERY wet trip.
In the dry, I oil the chain as soon as it starts getting noisy, more often if possible, which amounts to at least every 100km and definitely before every long ride.
It sounds like the whole setup is just gunked up with old grease and muck. Give the whole thing a thorough clean with (insert favourite cleaner here) and a stiff brush. I use kero for all that sort of cleaning but if you do a search, you'll find there are more options than cyclists so rather than refight the war here, do a search
You could pull the chain off and the wheel out and do the whole lot that way. Go right ahead if you're that way inclinded and you'll get the whole thing cleaner, but you can do a perfectly adequate job leaving the bike intact.
The pulleys in the derailleur do gum up and do need oiling as well, so include them in the clean and lube. This is probably where your problem lies.
Do all that and see what happens. Even if there is another problem, you won't be able to diagnose it while the drive train is dirty, so no matter where this ends up, the first step is a thorough clean and lube.
Have fun.
Richard
I hate cleaning bikes
I had a good bike ... so I fixed it