Charged A/C cycling at speed
 

Hello, recently I rebuilt my a/c system, and its blowing nice and cold now, but I noticed after the system has been running for about 5-10 minutes and I am at highway speeds it seems to cycle a lot, and I have a 4 cylinder so it's definately noticeable and kind of annoying. Only seems to happen when I'm doing about 60 in 5th at around 2200-2600 rpm. At idle I am running around 40 psi at the accumulator, and when I rev the engine up to 2500 rpm it gets down to about 28-30 psi. Anyone got any ideas?