[j-nsp] MX issue

[R LAS]

Strange issue.

Let put it simple, my customer have a network like this:

MX1 --- wan --  MX3
  |                          |
MX2 --- wan --  MX4

MX are MX480.

Pings from MX1 to a vip address active on MX4 are 40% lost.

A telnet from MX1 to a vip address active on MX4 takes 20 to 50 seconds to open the first time, the second is faster.
I suspect asymetric routing but it's still not clear (I do not manage the customer net).

I'm told that passing through traffic do not experience any kind of issue.

I'm thinking to ddos protection but still not got the output from the customer...
Any other idea about the issue ? 

Greetings