Thirty years ago, I purchased a Raleigh Seneca Mountain Tour Bike. I think it is a modest, basic mountain bike. I used it to get in shape but hung it in the barn after about three years. I am now 72 and want to get back in shape. I'm telling you this to let you know I'm unlikely to be a competitive mountain biker. I had the local bike shop mount new tubes & tires, and tune up the bike. I have one problem. I don't like the Suntour XC Sport 7000 rear derailleur. It tends to skip rear chain rings when going up hill. My rear speed cassette has six chain rings, the front crankset has three sprockets. Would it make sense to invest in a more recent rear derailleur (and front derailleur) or should I simply clean the rear cassette chain rings and derailleur, adjust everything, and quit griping. What do you think?