Two ICX7150 Stacked with Strange Route Behaviour Overnight

[Eric]

Have two ICX7150 stacked and use the latest Router firmware.

Stacked with two SPF+ 10G cable. With default settings since I still not figure out how to access the Web interface to manage.

Then I have a UXG-Pro as Gateway and DHCP Server, connected by SPF+ to ICX7150.

All works great during the day. Not sure because of low traffic at night, or just too boring. 
Next morning, few or major amounts of device/client/user will experience slow internet with symptoms of loose packages ( if PING).All I need to do is move SPF+ cable of UXG-Pro from one ICX7150 to another ICX7150 (no matter which one is Master).Then after some seconds, all back to normal. (sometime still need to do it again, to move cable back to original port)

Very confused and simply swap cable can solve the problem which I guess might be routing stuck somehow.Since I cannot write a script to physically move the cable around, so its very inconvenient.

Please help.