IAX Trunk unreachable

I have a client with Asterisk 13.28.1 connecting via IAX trunk to my voice core running Asterisk 15.4.1

Every now and then the clients IAX trunk will show unreachable on the Voice Core but it shows reachable on the clients system.

Only way to fix the problem is to unload chan_iax2.so and load it again on the clients side. I suspect it might be iax timing source issue but not sure as to how it can be resolved.

Any assistance will be appreciated?

Regards

IAX2 is very few used, a and if is source code issue system upgrade will be your only option, have you consider use PJSIP or is there any particular reason for using IAX2?