BGP session flapping

my customer is peering with 2 ISP’s and has configuration with 2 BGP routers mutually connected with iBGP. routers are reciving full tables from ISP’s and has 4 byte AS (XXXXXX). recently we noticed that BGP session between customer and ISP2 is constantly flapping each 3 minutes. logs didn’t show anything useful %BGP-4-MSGDUMP: unsupported or mal-formatted