Cannot use either the Desktop or Web-based app with RingCentral for inbound/outbound calling

I just upgraded my computer last week, and it hasn't worked correctly since. I've deleted and reinstalled twice. I used the web-based app, as that was suggested in the forum, but I get the same results. It is slow to the point of being unusable. Outbound calls NEVER go out, and Inbound calls ring to my PC, but I can't answer. It seems like general system slowness, but this is the ONLY app experiencing the problem. My IT team is stumped.