Coverage lost at times, Downloading a random file?

Hey there everyone! Ive got my Samsung Galaxy S3 and everything was working great until recently. I often times have full coverage and others none. These are in areas where I know I have always had great coverage. Also the phone is downloading a 4.jpg image all the time. Once I try and open it, it tells me there is no such image. The only time I can use the phone successfully is when I am on the wifi with wifi calling. Im on t-Mobile by the way.