Can any one help me, I have a1999 Hyundai Excel 5 speed, the gear box is slipping out of 5th and 4th gear mainly when on the highway. It is getting very bad and have been quoted around $900 for a second hand one fitted but am worried the same thing will happen and have been told it will be a lot more to fix my box. Has any one had the same trouble? if so what did you do?

 

Yes, sounds like bad synchronizers in the transmission.
However, usually it is opposite gears, like 1st and reverse, or 3rd and 5th.
I'd get a second opinion. And I certainly would not install a used transmission as they (I presume) would have no warrenty.