php with .html extension in iis5

i tried to make .html files run php with limited success (i have to...i'm migrating from a unix server with a ton of .html php files)... i set the isapi application mapping (i'm running php as an isapi extension), and yet i'm getting totally inconsistent results... sometimes, on rare occasion, it works. most other times, the screen's blank, but a view source shows ascii gibberish. the rest of the time, php doesn't work and the files are shown without any processing. help would be greatly appreciated. thanks. -abe wong