Mitsubishi Repair: wont stay in gear, chiltons manual, shift linkage

Question
My 98 Eclipse, 2.0 GS, manual.  Won't stay in 5th gear during
highway cruise.  The shifter will slip out into the nuetral
position.  Also 3rd gear really grinds too.  What can I do at home
to fix this?  I have several other cars to drive and do not want to
waste any money if I can do this myself.  Kinda like the challenge  
of doing it myself too.
Thanks,
Chip

Answer
Chip,
You probably have a loose or worn shift linkage.  These are the cables that go from your gear shifter in your car to the transmission shift and select levers on your tranny.  

If you remove the shifter boot on your gearshift, you'll be able to see where the cables are attached. And if you remove your air fitler housing and intake tube from under the hood, you'll be able to see where the cables connect to the transmission.  Make sure the connections are secure and have not slipped at either of these points.  

Other things to check are clutch fluid and transmission fluid levels.  The clutch fluid level is easy to check from the resevior attached to the master cylinder on the firewall.  But the transmission fluid can only be checked by raising the car levely, and removing the fill plug.  The fluid level should be at the bottom of the fill hole.  If you have not replaced your transmission fluid within the past 30k-45k miles, it should be done.  Use only the recommended fluid as noted in your owner manual.

But if you cannot find the problem in any of the items I've mentioned, the problem could be a worn or broken transmission mount.  That would need to be replaced.  I'd also recommend getting a service manual for your car if you're interested on maintaining it yourself.  Most auto parts store have a Haynes or Chiltons manual that is available for the Eclipse.
Good luck!