we just had recently exact the problem with a customer and the TCP Idle Timer. Unfortunately it was a really annoying situation for the customer. Microsoft writes it is solved...but it is not..or maybe not in all cases.
Anyway...the default TCP Idle Timer of the MX is 300 seconds (5min).
Via a support case you can change it to either 3600 or better 7200 seconds. Which is 60 min or 120 min.
It helped in our case and the customer is happy again.
hope it helps