I found this at
www.vfaq.com....hopes it helps.
This is a symptom of a serious problem, at least on 1G DSMs. Shifter movement is not generally normal in DSMs. Having the shifter move around, especially in 5th gear, or popping out of gear (even once or twice) could be an indication of a loose retaining nut inside the transmission. Several 1G DSMers have had the rotten experience of having this nut come off completely, and consequently blasting a hole in their transmission. This problem only affects FIFTH gear - gears 1 through 4 and reverse do not appear to suffer from this difficulty.
Those who are uncertain must read this post and this post, both by Paul Lyons, and this one by Ashok Babu, who had his transmission fail on him. Don't take a chance on this one, since the inspection procedure is easy.
This problem does not appear as common on 2Gs, and at least one owner has reported that there was a loose nut in his shifting assembly that was causing the shifter to pop out of 5th. This is NOT the same nut that is involved in the 1G problem.
Is yours a 1g or 2g?