Constant wifi dropouts

I was sent the new white tower modem upgrade as per Xfinity claiming it was faster and there was no charge to upgrade. Received it on 9/27, installed and connected it on 9/29 following the directions via the phone app. It took more than an hour of rebooting both the modem and my PC before it finally connected to the internet. Since then, the wifi randomly drops out for 1-3 minutes with no warning yet the modem has a continual white light. Advanced Tech Support looked and says that the signal is strong and saw no problems. They scheduled a service tech to come out (which I have to pay for) on 9/30 but I cancelled due to the modem suddenly working properly. Then today (9/30), it's back to dropping out off and on. ATS had said that the original connection problem was due to the system still seeing my old modem and he did the required swap. When I called today, they say they can't swap it back to my old modem and a tech will need to come out. Is there a solution to this or do I need to return the new modem? I never had these problems with the old one and it very frustrating as I do a lot of work online and my home security system is connected to the wifi. It also keeps dropping off and is not easy to reconnect.