High CPU, 100% retransmissions , Drop modulations

Hi guys, can anybody give a tip for this case. 

I have a ptp link with two non gps epmp1000 radios with the last version ( 3.5.2)

As you can see , there is a time where i have almost 100% of retransmissions , drop of modulation leves for a while and 80% CPU use.  all this with a very low of use of the link.

Costumer says that he has experimented ssh drops and annoying voice quality on calls.

Thanks in advanced

Can give more details about the link... distance, modulation level, Tx power... etc

Hi , is a 4 kms link

tx 12 dbi

antenna 25 dbi 

MSC 15 almost all day 

epmp 1000 both sides.

10 mhz 

today i changed the frecuency channel , performance is better but same behavior with retransmissions