There are more causes that you can have. Amn idea can be to do a fast test by lowering the MTU network setting on External Port (Advanced Port settings) in your Pulse central appliance from MTU 1500 to 1320 for example. Be carefull in production cause this will auto restart network services and give short disruption for all connected clients! Other thing is try to adjust setting in your ISP home firewall/modem. Keep firewall enabled! But try tot disable ip-flood-detection or other special things to see if this has a positive effect.
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