Something we learned a long time ago is that spark plug wire boots need to be "burped." The boot has a bit of trapped air inside from the installation and in some cases this trapped air expands with underhood heat, pushing the wire right off the distributor cap. The easy way to prevent this is to simply "burp" the wire boot with a thin, blunt tipped tool. We use a cotter pin removal tool, as seen here. Just insert it carefully and give it a twist to pull the boot away from the cap a little and you'll hear the air escape.