1985 model, first year of production.
Tange 1 frame with nice sloped crown fork.
Shimano 600EX was OEM, and I'd find another 600EX rear derailleur for it.
If it's jumping out, probably a worn cassette or chain. However, that may be a freewheel, so double-check.
There are 6-sp 600 freewheels out there, as well as 6-sp UG cassettes out there, so should be easy to find.
Should clean up quite well, and getting the wheels trued and tensioned will be like getting new wheels.
Given a proper cleaning, then repack of the hubs, headset and bottom bracket, then lube and ajust cables and shifters, you'll have a killer ride that is way beyond a $180 bike. Easily one to invest in.
I'm building one now, same year, different color.
I'll put $500 into by the time I'm done.
Won't lose a second of sleep over it.