pump pressure

[poolhelp101]

I have an inground 21,000 gallon pool with sand filter. i am starting to have pressure problems when vacuuming. it starts off fine and vacuums well, then about half way thru, the pressure gage drops to less than 5 and it won't vacuum anything up. i turn the power off the pump, then start it back and the pressure will be fine, then start to vacuum and the same thing happens again. the pressure is fine any other time and does not drop. could you please advise as what is wrong or what i need to do to correct the problem. thank you.