wifi Crashing

I am having problems with my Broadband crashing about 2 to 3 times a week at around 21:00. Virgin have sent an engineer and he could not find anything wrong. Things were ok for around a month after he visited.  Now the problem has returned when I do a test it comes up with a message about the being intermittent strength. I have rebooted the router sever times which makes no difference after around 30 minutes all is good again.

Does anyone else have a similar problem and is there a way to fix it