Krabi is one of the TG designated international airports which should allow you to pass through immigration in Krabi and be identified as a CIQ passenger. This means that upon arriving at BKK, you can proceed to the CIQ transfer desk, located on Concourse B, just beyond the intersection with Concourse A. There should be TG staff there looking for all passengers with the CIQ sticker and directing you where to go. From there I suppose you'll need to find the CX international transfer desk, which I believe should be straight ahead, right after you pass through the CIQ transfer point. Should be a piece of cake doing it that way, but even if you didn't go through immigration in Krabi, 1h25m should still be plenty of time.
Regarding your transfer to PG, if you are forced to go through immigration and collect your bags, then you will have to pay the 500 Baht (raised to 700 Baht as of Feb. 1, 2007), departure tax as you pass back through immigration. Try to see if it's possible to get your bags interlined as it will save you some time and money.