When the transmission is manually shifted from gear to gear, there should be a solid, positive feel of engagement as the shift arm is moved thru the gears.
If it feels sloppy then check to be sure the external linkage isn't loose at any connection. If the linkage does not seem to be the problem, then the issue would be in the valve body. The valve body is located under a cover on the driver's side of the transmission. The shift linkage is attached to it and the linkage uses what's called a detent lever to physically engage each gear when manually selected. If that detent lever is misadjusted or worn then it will allow the transmission to slip into another gear.
Start first with checking to be sure the linkage from the column to the transmission is properly adjusted. The linkage at the column on the 54s is all base (pot) metal and is known to wear on higher mileage cars.