Udp failure


Hi, i have strange problem and dont know how fix it, and i think it bug on kvm virtualization with qemu
lets try explain what a problem:
i have server in datacenter one of my clients have bots that sending udp packets to his server in another datacenter
the client send udp packets 24/7 to his server from my server, now after 2-3 days the udp packets begin failed
and not come to his server, then i give him other ipv4 address and all back to work again for next 2-3 days
after then it failed again and i need give him again new ip. i talk few times with datacenter they say not block anything and i even not behind firewall (directly to internet with no kind protection), he try with his bots from other server and it works to him like a week with no problem then no issue with his bots. now i try figure from where it coming and i think the qemu have problem with udp packets when you send them 24/7 and i think it bug mybe, can someone give me advise what can i do here?