IPSec N2N short interruptions

I have an IPSec net-to-net connection. Every now and then I have short interruptions of a few seconds:

64 bytes from 192.168.77.1: icmp_seq=24 ttl=64 time=18.8 ms
64 bytes from 192.168.77.1: icmp_seq=25 ttl=64 time=15.4 ms
64 bytes from 192.168.77.1: icmp_seq=26 ttl=64 time=16.3 ms
64 bytes from 192.168.77.1: icmp_seq=37 ttl=64 time=16.7 ms
64 bytes from 192.168.77.1: icmp_seq=38 ttl=64 time=19.8 ms
64 bytes from 192.168.77.1: icmp_seq=39 ttl=64 time=20.4 ms
64 bytes from 192.168.77.1: icmp_seq=40 ttl=64 time=17.2 ms
64 bytes from 192.168.77.1: icmp_seq=41 ttl=64 time=18.2 ms
64 bytes from 192.168.77.1: icmp_seq=42 ttl=64 time=16.3 ms
64 bytes from 192.168.77.1: icmp_seq=43 ttl=64 time=17.2 ms
64 bytes from 192.168.77.1: icmp_seq=44 ttl=64 time=16.0 ms
64 bytes from 192.168.77.1: icmp_seq=45 ttl=64 time=18.4 ms
64 bytes from 192.168.77.1: icmp_seq=46 ttl=64 time=21.4 ms
64 bytes from 192.168.77.1: icmp_seq=47 ttl=64 time=17.8 ms
64 bytes from 192.168.77.1: icmp_seq=52 ttl=64 time=19.6 ms
64 bytes from 192.168.77.1: icmp_seq=53 ttl=64 time=20.5 ms
64 bytes from 192.168.77.1: icmp_seq=54 ttl=64 time=17.0 ms
64 bytes from 192.168.77.1: icmp_seq=55 ttl=64 time=17.3 ms
64 bytes from 192.168.77.1: icmp_seq=56 ttl=64 time=18.4 ms
64 bytes from 192.168.77.1: icmp_seq=57 ttl=64 time=15.3 ms
64 bytes from 192.168.77.1: icmp_seq=58 ttl=64 time=16.6 ms
64 bytes from 192.168.77.1: icmp_seq=59 ttl=64 time=18.1 ms
64 bytes from 192.168.77.1: icmp_seq=60 ttl=64 time=18.0 ms
64 bytes from 192.168.77.1: icmp_seq=61 ttl=64 time=18.5 ms
64 bytes from 192.168.77.1: icmp_seq=62 ttl=64 time=16.6 ms
64 bytes from 192.168.77.1: icmp_seq=63 ttl=64 time=17.2 ms
64 bytes from 192.168.77.1: icmp_seq=64 ttl=64 time=17.6 ms
64 bytes from 192.168.77.1: icmp_seq=65 ttl=64 time=19.0 ms
64 bytes from 192.168.77.1: icmp_seq=66 ttl=64 time=16.2 ms
64 bytes from 192.168.77.1: icmp_seq=67 ttl=64 time=16.3 ms
64 bytes from 192.168.77.1: icmp_seq=68 ttl=64 time=19.3 ms
64 bytes from 192.168.77.1: icmp_seq=78 ttl=64 time=19.5 ms
^C
--- 192.168.77.1 ping statistics ---
78 packets transmitted, 50 received, 35.8974% packet loss, time 77721ms
rtt min/avg/max/mdev = 15.256/17.849/24.612/1.609 ms

What can be the reason?

Thanks in advance. K.