Wow, that's weird. Never heard of AJAX requests being cached. I would think that the caching must be happening on the ASP side of things, except that I haven't heard of that, either.
Ehhh...there are worse answers than needing to use a random number for a cache buster.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.