1992 Dodge Stealth 5th gear pops out

My mechanic replaced 5th gear due to it popping out at freeway speeds recently. I only used 4th gear after it popped out into neutral. He installed used 5th gear, slider, pin and synchro, popped out again at 72 mph in cruise control after 250 miles of freeway driving. I paid $190 for those 4 replacement components which I understood were to be new but turned out to be used after the second failure. He has checked the motor mounts and linkage cable adjustment and says they appear fine and he can't see any adjustment possible with the 5th gear cable/linkage. Is this correct? Can motor mounts be worn or faulty though intact? What next, would new components be the difference here? Base model Stealth has been babied since buying in 02 with 98,000 miles. Transmission fluid changed approx. 10K before problem. Slave cylinder and clutch have been replaced during this latest procedure. I'm wanting to sell the car but obviously can't in this "unfixed" condition and can't justify the cost of rebuild or transmission swap at this juncture. Should I then just sell it as a parts car, all else is perfect condition and original, inside and out. (I drive like the 60 year old that I am-never hot-rodded.) Too late (after $500 sunk costs) now but appears I should have taken it originally to a transmission shop since my mechanic doesn't seem that knowlegable about trannys. He gave me conficting information early on about whether he was getting new vs. Used parts. So I don't want to sink anymore money for new parts if this isn't likely to correct the problem. He says he can see the cables and gear shift move when he accelerates in 5th. No such issues in the other gears. No noise, grinding or difficulty shifting now or prior to this problem showing up.