If you're talking about the solenoid on your starter, hold on before you pull it. If it starts every time you jump the starter with a wire, that usually indicates the start solenoid is good.
If it won't spin every time with the key, however, it may not be your starter. Keyswitch contacts go bad and start getting unreliable, and there is a relay switch between the key and sarter that could be hanging up. If it's a G-series machine with the fuse/relay box down by your feet, the relays have been known to work themselves loose from the sockets, and that would cause your problem.
I've found it's generally less expensive to have the starter rebuilt at a shop than buy a new one.
Good Luck.