Here's an idea. First, use an algorithm rather than a random selection. Take some things like the current time and the browser's IP address and apply something like a hashing algorithm to generate a number from them. Use that number as an index into your file, wrapping as necessary. Then, add offsets to the SSI calls, e.g. /my/cgi.pl?offset=1, /my/cgi.pl?offset=2, etc. In each call, add this offset number to your computed index. If showing the ads in sequence is a problem, multiply the offset by something, but keep it small enough to avoid wrapping around and possibly showing the first one again.