This is caused by either the shifter not going all the way into gear, as stated above, or the pilot bushing, but more than not, the end play in the mainshaft/input shaft is too large, allowing the synchro ring and slider to not engage the input shaft deep enough to hold the gear in. This will usually show up in decelration, not cruise or acceleration.
The fix is to pull the gearbox and check the front to rear play on the input shaft, with the outlut shaft pulled back all the way in the case/extension. The input shaft can be shimmed between the front bearing and the thrust face of the input shaft, but the damage to the synchro blocker and input shaft may be too far gone to completely fix it from the outside.
Please note, it could also be a detent problem on the side cover or a bent or worn out shifter fork/synchro slider groove not engaging high gear deep enough as well.
Stop screw on shifter could be too far in, set for a more shallow second gear stop. Worth checking as well. Set screw for deepest gear engagement between sets 2/4 and 1/3.
[This message has been edited by IgnitionMan (edited 08-29-2000).]