They replaced the box two times they were there,.. is the box a modem/router combo? or do you have a separate router for wifi? Just saying because I have found that chances are after something has been replaced one or two times and the problem still persists, then most likely, the new equipment is not the problem.
If it is a combo unit, then the problem may lie within your computer itself... maybe a bunch of stuff running in the background that doesn't need to be... little things like that, or even the age of the computer and the condition of the built in wifi... if you have an ethernet port, maybe try connecting a cat5 or 6 cable from computer to the modem unit directly and see what happens.
if you have a separate router for wifi, then the problem could be within that, and a trip to walmart to get another one may cure that.
Just some tips to check out.