Hyundai Repair: clutch, gas millage, hyundai accent

Question
Recently had a clutch replaced in a 98 Hyundai Accent,  In  a about a week it began to pop out of 5th gear.  I was told this was some type of transmission problem,  I on the other hand think there is some type of linkage that can be adjusted. (I noticed that when it popped out of gear and I would reengage it seem as though the gears were not fully mesh.  
The 5th gear problem has now manifested to when I had attempted to get into 5th when I disengage the clutch grinding of the gears will ensue.
Do you have any suggestion as to where the problem may lie.  I hate being without 5th gear because I do an extensive amoount of travel with my job. Not to mention what appears to be a significant drop in gas millage.

Answer
If the problem exists in all the gears where you shift forward (1st, 3rd, 5th), then there may be an issue with the shifter or shift cables.  Similarly, this may be the issue if the problem occurs in all gears.  But if it's occurring only in fifth gear, the problem is almost without question inside the transmission.  Any adjustment made to the cable would affect *all* gears.

The (almost) good news is that the transmission doesn't have to be removed to repair problems with 5th gear, its synchronizer, or the 5th-reverse shift hub.  These items are underneath a small cover on the end of the transmission and can be replaced without creating an enormous repair bill.