Strange noise with tdm400p and zaphfc on via epia

I have a tdm400p with 2 fxs modules on it and a hfc cologne isdn board on a via epia with c3 1000 processor.
With only one of the above boards everything works fine. When i use all the two boards, from one of two analog phones i hear a strange cyclic noise ( every 5 seconds ). The two boards are on the same pci slot but have got two unique irqs ( one for each board). i have got the problem only every 5 seconds, for the rest of the time i hear the remote caller perfectly.

The boards are connected to the m/b with a pci reiser card, so with it i can use one pci slot for two boards.

i need for this kind of solution with reiser card

i hope somebody help me