Losing pressure after a few hours (suction leak?)

[dunning]

I have a hayward perflex EC50 DE filter. After 4-5 hours it loses almost ALL pressure. If it was outputting 50 gallons per minute, after 4-5 hours it feels like / appears to output less than 2-3 gpm - it is far weaker than regular garden hose pressure would be - you can almost completely stop the flow with your hands if you wanted. I suspected a suction leak but couldn't find any.

Simply turning the filter on/off for a few seconds, it fixes the problem for a very short time, like 5 minutes.
Bumping the handle and opening the drain valve for a tiny bit fixes the problem for a bit longer, like 1 hour or so.
Doing a backwash fixes the problem for like 3-4 hours.
Doing a full cleaning (taking the whole filter out and cleaning it) fixes the problem for about 4-5 hours.
These symptoms make it seem like the filter is getting dirty and needs cleaning. But the filter doesn't appear to be that dirty.

I figure if its a suction leak, wouldn't it consistently lose pressure steadily at the same rate? I am going through far too much DE earth trying to figure out why this is happening. The filters still appear to be in good shape, and no DE powder leaks into the pool. Am I overlooking something?

[Denniswiseman]

With a suction side air leak you get air bubbles in your returns. Does the pump pre-filter have a lot of air in the top?
It could be that your filter is getting clogged
Let's have your numbers (CYA is very important) Use a pool store if you don't have a FAS/DPD kit
FC:
TC:
pH:
TA:
CH:
CYA: