I have a 1999 TREK 6000. All parts are original. The right shifter is not working properly. It will shift from for 8th to 4th but then it just quits. You can push on the shifter but it just goes all the way and "bottoms out." It clicks fine 8th through 4th but then it just quits grabbing. I took the cable loose from the derailleur and it still does it. Is the shifter itself broken? Any help would be appreciated. Thanks.

It's impossible to diagnose without seeing/feeling it, but given the age, cross your fingers and hope that it's just dried lube or crud. Flush it generously with WD-40 or a similar light solvent/oil product. While it's flushing work the lever through the range and see if things improve.

I like to do this with the rear wheel off, or reversed, so I don't have to worry about pedaling while working the system back and forth through the gear range. If you live right, you might break lucky and the flush will solve the problem, Then dry it off as well as possible, and add a tiny amount of light oil to keep it lubed.