I just replaced my front shifter on an early 90's, 21 speed mountain bike. Now I'm trying to adjust it and it keeps jumping all the way from first gear to third with just one shift. Does this mean it's not compatible with my derailleur or am I just doing something wrong?

Shifter is a Shimano Altus SL-M310 and the derailleur is a Shimano 200 GS.

Shifter and derailleur are compatible.

Most likely it is a problem with setup.

It could be a problem with the angle that the derailleur is clamped on the seat tube (needs to be 100% parallel with chainrings when viewed from above).
It could be a problem with cable routing, although this is sort of unlikely... if the cable is routed wrong it may change the way the derailleur works.

Alright I got this figured out, just had too much tension on the cable. Still not sure how that would cause this particular problem, but loosening it fixed it, just in case anyone else runs across this problem.