Proxies, Cache and other Strange Happenings

Hey, I have an rather annoying problem. I have this page that passes a bunch of variables via a url string to another page which has a few if statements in it and displays different responses depending on the the variables sent to it. That's all fine but what happens is on this one isp ( I have simulated this on four others ) the correct url string appears on the top window of the browser but the wrong messages are displayed for the variables sent to it, even stranger hit the refresh or put the cursor in the url bar and hit enter, it will display the correct result. As I mentioned above this only happens on 1 out of 5 isp's ive tested it on so it doesn't look like a coding problem, I feel that it is some weird proxy thingy. I have cleared the local cache etc.. but still the same result, it just refuses to display the correct information until the page is refreshed?? I have also tried the: header ("Cache-Control: no-cache, must-revalidate"); The whole site uses variables passed via the url and conditional statements but this one page on this one isp is having problems, I must say this has me stuffed??? Has anyone out there had any similar problems? Regards, Joseph