I had the same problem with my old sand filter. I would vac and after a few minutes, I could see dirt returning to the pool. I took the valve off and checked the sand bed, it seemed deep enough and there was no channeling. I backwashed it and continued vacuuming. After a few minutes dirt was returning to the pool. I was also having a problem with water escaping from the backwash hose when the valve was set to "recirculate". I knew the spider gasket had blown. At the beginning of this summer, I emptied the sand and checked the laterals. None were cracked. I put pea gravel in to cover the tops of the laterals and added the right amount of new sand. I replaced the entire valve with a new one because the old one was developing hairline cracks. No big deal. Everything seemed to be working pretty good and the tank developed a fracture in the neck below the compression ring for the valve. I also had another problem with water leaking from the backwash hose again when it was set to "recirculate", and this is from a new multiport valve. At this point I could replace the tank for about 300 or so dollars and replace the spider gasket. I had enough at this point. I decided to check on what was available and went with a cartridge filter and a brand new pump for less than the "tank only" option. Plus, I've only cleaned the cartridge once in the middle of summer. I won't clean it again until I close the pool sometime in October. And just for fun, I disassembled the new multiport valve I was using and guess what, there was nothing wrong with the spider gasket, it was pristine. Go figure.
I believe there is a possiblity your pump is too large for the filter. Some of the other members here might have a different opinion, especially the ones who use sand filters exclusively and have not had any problems.
