2000 Plymouth Neon manual transmission problems?

This thing shifts out of gear when in 5th and crusing down the interstate. The transmission was rebuilt one year ago and new cables installed, so the bill says. It looks to me like the shift cable bushings are bad, would this cause it to shift out of gear? I’ve always thought that when a manual transmission is engaged in a gear it wouldn’t just slip out of gear by it’s own accord. Only thing I can think of is the syncro’s going bad inside of the transmission or the shift cable needs to be replaced. Please help, it’s my girlfriends mother’s car and I want to keep my girlfriend.
Thanks in advance.