Rans Rockst (Retro rocket) Rans Enduro Sport (Retro racket) Hase Kettweisel Tandem (redundent recumbent), Merin Bear Valley (The gopher).
Springs never get stronger with age, they can only weaken. If your rear derailleur ever worked right the spring doesn't need to be weakened.
With friction shifters the most common problem is that the shifter doesn't have enough friction. That is usually easily solved by tightening the screw or sometimes a wing nut that holds the shifter onto the bike. Obviously spraying a lot of lube onto anything that works by friction, like brakes or a friction shifter, isn't a good thing to do.
Rarely the shifter has been disassembled and reassembled incorrectly at some point during it's life. If you think that might be the case and the front shifter works right, take it apart to see how all of the little washers etc. are lined up.
Had exactly this problem with the '83 Cilo I'm fixing up. Took the seldom-used shifters apart, cleaned 'em up, and put 'em back on the bike. The right one wouldn't hold. Found I had put one of the washers in wrong.