Gears Slipping Out at Speed

[flashlight]

It started last night when a friend and I were headed out fishing. His car was doing fine when we were in 5th gear and then all of a sudden to just slipped out of gear. So he switched it back into 4th and then tried for 5th. It was not going to happen no matter how high the rpms were. So we pulled off to the side and just did a manual change through all the gears with the car off. On this part of the test everything worked until 5th. So we ended up calling the trip off as we really didn't want to get stuck in an area with no cell service in case we needed to call for help. On our way back home the car proceeded to "slip" out of several other gears. So now it is slipping out of 3rd, 4th, and 5th. In order to make it home he had to shift into the gear that he wanted and then have his hand on the stick so that it couldn't slip out. The odd thing is when we got back into town the 3rd and 4th gears were working like normal. Can anyone help us in figuring out what the issue(s) are? And what the heck is going on? He drives a 95 volkswagon jetta, manual transmission, with about 231,000 miles on it. As far as the engine size goes I don't have a clue as I haven't been helping/ working on that. Thanks.