Wireles mini pci card is not workig properly

Hi!
I purchased one RB 433 routerboard with Level 4 license and two mini pci cards. The one is compex (2,4 Ghz-5Ghz, but I use it on 2,4 Ghz) and the other is ubiquity (5 Ghz). My problem is when I make a simple configuration, I bridged together all ports and the ubiquity 5 Ghz card is working peoperly but the compex card is very instable. When I connect with My MacBook on the 2.4 Ghz card and start a download it starts with max 9 Mbps but after a few seconds it slows down to 512 Kbps. On the 5 Ghz side I don’t have this problem (I have a 20 Mbps Fo Line).
Anybody some ideas?
The card have a malfunction or my config is not god? (I use simple bridge’s on each ports)