RT-AC87U very poor performance

Hi All

I am new to the forum. I hope someone can help.

I have an RT-AC87U router connected to my virgin media modem on a 200mb broadband connection.
Hard wired, I can occasionally get around 200mb download speed. With wi-fi it is around 30 - 45mb and is not a smooth download. It also drops the connection sometimes although all devices get a pretty good signal strength.
I had a look on the web for some info on this. I tried with QOS on manual and auto and also disabled but bandwidth is...

RT-AC87U very poor performance