Wireless Bridge lost Signal

Hello,

i have a very frustrating Problem with a Mikrotik Bridge.
I have two 433 Routerboards with two Yagi Antennas and 2,4 Ghz.
The Wireless Cards are R52.

Sometimes the Singal ist completly down an can´t connect to the other Routerboard.
The Signal ist Completely lost and then it came up with a very Bad Signal ( - 75 dbm ) after 5 Minutes the Signal is going better to - 55 dbm.

We test everthing with the Antennas but nothing is work.
Is that a Complication with a other Wireless Signal ? Someone have a Idea to fix this Problem ?

Greetings

What information does a frequency scan give you?