Internet going in and out sporadically

I need help! My company has been having issues with our internet for about a month now. We have a firewall for our franchise which we have to have. Our ISP is ATT. They both blame each other for the issue. I thought it might be a bandwith problem. I had to computers ate up with viruses. I cleaned them and got them working correctly. Internet seemed to be working fine for a day. Then this afternoon it dropped in and out again. Help!
 
Set the gateway router to have an MTU of 1492. This stops packet fragmentation and the necessity of reassembling them.

Also verify that router has the latest firmware.
 
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