I have to FF800 linked by ADAT 1 and WC. All settings are correct in sample rate and the two units have the same drivers and updates (sorry I don't have which driver version but I installed them this winter downloaded from the RME site)
My computer is a MacPro with 4 Gb RAM and Quadra 2.25 processors.
I run Cubase 4 with the latest upgrades and updates. 

I had my units for 3,5 years and they been working perfect til' now.

I'm having this strange problem that occurs after 20 minutes of use.
The second unit gives out crackling sounds and sometimes distorded noise. At the same time I see that the  WC and ADAT LED's are flashing, so it seems like they are losing the connection and that what makes the crackling sound. When I look at the mixer the sounds appears on channel 13-20 (2nd unit). The first unit is silent.
The problem is coming during record, playback and even if I just start them up. And always after approx 20 minutes.

At first I thought it might be overheating so I spread the units apart in my rack, 1 unit in between, but it didn't help at all.

Have anyone else heard of this problem or in best case know what parameters might fix it?

I'm very grateful for help I can get. If you need to have more information regarding set up and other stuff I am glad to provide it.

magnus