Or schedule your cron job for :00, but add "sleep `jot -r 1 0 3600` &&" to the start of the command. (jot is a BSDism, but I assume you can do the same with GNU seq.)
This might be platform dependent though, I can't find any standard RAND_MAX in bash so it's difficult to make this work everywhere.
sleep $(( 0x$(xxd -l2 -p /dev/random) * 3600 / 65536 )) s/\//\\%/
yeah. sleep $[RANDOM\%3600]