iPhone 4 goes from full bars to "searching" all day long..

I have an iPhone 4 with Virgin Mobile USA and recently my phone goes from full bars to "searching" and then I get full bars again, and this just repeats all day long. I have tried factory reseting the device, and reseting the network settings, I have toggled airplane mode off. The only thing that seems to sort of work is if I turn my data off then my signal will not drop out to "searching" I honestly really don't know what to do, This has been happening for over a month. I've contacted my carrier multiple times and they say that it isn't a problem with the network that it is a problem with the phone. Does anyone have any advice? Thank you for your answers.