I don't wish to come across as the harbinger of doom, but I'm afraid your gearbox is probably on the way out.
If you look through posts on this forum from about two years ago there is a thread regarding this exact problem. Its a while since I looked at it, but it alludes to the early KIA gearboxes having been maufactured by another company (Hyundai or Daewoo I think) and that they are so poorly constructed they're only good for about 70,000 miles.
Mine lasted approximately 70,003 miles before 4th started to make a hearty crunching noise every time it was engaged and by 70,500 3rd gear decided to go walkabouts and has been located only occasionally ever since, though its always a treat to meet up with it again.
The GOOD thing about all of this is that my motor is now up to 96,000 miles and being the kind of person who doesn't change things until its absolutely necessary I've never bothered to change the 'box. In the meantime I've continue to use the car for holidays abroad anduse itdailyfor my job and I intend to do exactly that until the 'box completely crashes.
My Caren 2003 has cover 95000 miles and yes the gear box is not the best in the world but as mentioned at the right speed it goes in 4th with out making a sound and slips in with out a problem. as mentioned I did change the gear box oil which has made a difference.
Just out of interest and in the absence of a suitable operator's manual, where is the gearbox oil filler cap and drain plug situated on the engine? I think it might be prudent for me to top mine up as soon as possible.