Hotspot - IP Bindings

Hi All I hope someone may be able to give me a steer on an intermittent issue that I am having.
Board Name: RB3011UiAS
Version: 6.43.2 (Stable)
Factory Software: 6.35.3

I use built in radius server via UserMan and that is working well. However I have a number of PC’s that need to bypass the Hotspot and so their MAC addresses (NOT IP addresses) are entered in the IP/Hotspot/IP Bindings and type entered as ‘Bypassed’. This has been working very well for many months, however over the past couple of weeks the PC’s in the bypassed list are intermittently loosing connectivity to the internet as they cant find their default gateway. Sometimes the fault just gets better on its own and sometimes I restart the RB3011 which fixes the fault for another day or so but then the fault reappears. When the fault is happening, the Hotspot users continue to wok as normal. If I disable one of the PC’s bypassed status the PC can login and work as expected using the Hotspot. Where do I look please to try and identify this fault?

Any help gratefully received