1997 Volkswagen Jetta Bolts coming loose!

Ok so ive was driving my jetta and started to hear a little noise coming from the driver side so I pulled over and looked around the car and I didnt hear anything. So I attempted to pull forward a bit and than I heard a clunk. My cv axle bolts fell out, all 6 so I tightned them back down and drove my car another 500 miles checking the bolts. The other day I noticed they have walked out again. What would cause them to do this. Ive checked the price to replace the joint coming from the tire to the transmission. I was wondering if the area that comes from the transmission can be replaced?