Updating shifters on mountain bike
Next, gently move each derailleur by hand to check the bolts that attach it to the mountain bike frame.
Loose derailleur bolts might not cause a shifter to malfunction, but a loose attachment can affect gear changes.
I have an aluminum frame bicycle (mountain bike, 21-speed) that I purchased some time in the late 90s, it's marked "Skykomish Eureka" (though I can't find any evidence that such a brand and model ever existed), and it is currently outfitted with Shimano 300LX components.
I believe that the rear shifter is giving up the ghost, as sometimes the lower (up) handle will go all the way back without "catching" whatever it's supposed to be catching in order to shift the gears.
If the comments in the link are anything to go by, the frame is nothing special, so if I were in your position I'd be leaning toward a new bike.Deraillers almost never "wear out", at most they need new jockey wheels. What does get worn out is the rear cogs or cassette. Generally getting good performance out of canti's has more to do with how well you set them up rather than the exact model you use.