1-2 controller cards in system: How to initialize onboard controller first?

Question

I have an older mobo in my system whose Promise PDC20376 onboard controller sometimes causes minor problems when other external cards (bus is good ol' PCI) are present and the onboard one gets initialized last. Unfortunately, I can't see any way in BIOS how to change that sequence of initialization.
Let's assume I have one of the common "true" SATA controller cards installed like the Promise TX4. (NOTE to explain the meaning of "true": Sometimes you may also (mis)use a RAID controller card to add more drives. Though some do allow that, you may assume I am not doing that here, in order not to create yet more possible problem sources)
Whenever there is 1 or 2 hard drives attached to the external controller card, it can be observed that the ext. card will ALWAYS be initialized first, and then the onboard controller card.

Is the sequence in which this is done totally dependent on BIOS/board architecture or can there be taken influence somehow?
I guess it won't make sense to fiddle with IRQs in a random fashion, since it may take ages until I've found the "ideal" combination...