Robert;

I guess you've checked out the durso standpipe website, right? Like, if you have 1" drain, you should use 1 1/4" PVC to make the standpipe. I also use the thin walled PVC, which allows for more flow than schedule 40. The use of one size bigger than your drain does the trick. Also, try just taking the cap off, letting unlimited air into the pipe, and see if that stops the level moving. And, try switching pipes to see if it's the pipe itself or something weird in your drain on that side.

HTH