Have you removed the drive belt and checked all the pulleys for any play in them, or tightness? I'm wondering if you have a bearing going in something (besides the rods). Could be the A/C or waterpump.

Do you have any way to get a mech. oil pressure gauge on it to see what you are running? Maybe you're oil pressure is getting low when its fully hot and letting it knock a bit, then when you raise the RPMs it goes up and the knock subsides. Just throwin ideas out there.