Having high packet loss at night, could lowering MTU correct this?

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In the hours between 8:00 - 10:00 pm, my internet connection suffers severe packet loss (30-50% over 50 pings). I've been dealing with my ISP over the course of the past 2 months, trying to get a resolution to this. They swear up and down we are in a low demand area. But the problem ONLY occurs at these specific hours. Finally after 4 technicians and a network engineer working on the problem, it seems as if they have come to the conclusion "there is nothing that can be done"...

The tech lady that I spoke with this evening gutted up and admitted it's more than likely due to network demand load causing the problem.

In my desperation to fix this I've been researching possible fixes on my end, and one thing I have been reading about is MTU... Could this possibly fix the latency issue I have been experiencing at night? Any help or advice on the issue is greatly appreciated.
Again, the packet loss only occurs at specific hours of the night. During the day the network runs like a well oiled machine. If the problem were on my end, in my computer as you suggest, wouldn't it be a sustained issue?

The technicians have done a lot of line tests. Our SNR shows absolutely beautiful and well within normal operating limits. No packet loss, etc. But they can't be out here when it starts happening of course, because it's well past business hours.
No torrenting or anything of that nature. I play World of Warcraft at night, and during the day.

The game runs fine during the day, no packet loss or latency. Everything is super responsive. Then I can just sit and watch as latency starts to creep in as the evening approaches. It starts creeping up, things start becoming unresponsive, until its full blown unplayable.

I did ask them about data usage and Fair Access Policy type slow downs, my ISP does not do this.