What would cause a poppet valve to cycle?

[silverbullet02]

225 promax. It did this a few years back and then my poppet diaphragm started leaking and I replaced it with a used one I had laying around. I hasn't done it for years and started doing it again recently. Just mid range cruising around 3000rpm give or take, you can watch the water pressure gauge cycle from 10-15#-ish. With the offshore mid you can really hear it too. (still have 2 piece plates and all stock cooling) I'm going to replace it again, just curious what might make it do that. Weakening spring maybe or something hanging up in there?

[XstreamVking]

Check and or replace the rubber grommet that fits in the block portion these can cause problems like you describe

[silverbullet02]

Check and or replace the rubber grommet that fits in the block portion these can cause problems like you describe

I've already got a kit on the way. Thanks!