My Cox Modem keeps going down/several times a day

My Cox modem kept going down/up many times a day, finally had a Cox representative come out on 3/11 and things worked great for 3 weeks and then it started again every day. Every time I talk or chat with a support person all they do is send a new reset which only lasts for a couple of hours. I am getting so frustrated and want to know if anyone else has some answers. I do have the Cox 3 in 1 modem/router which worked great for 2 years and now has been replaced with a new one and still the same problems. HELP. Thanks, Sandy N