Wifi Trouble shooting

I've been struggling to find the source of EMI/noise that is disrupting wifi in a part of our building for the past week. Inquiry from the neighbours regarding newly added equipment/ potential wifi sources wasn't very fruitful. The noise level fluctuates and disrupts the wifi but at times it works just fine with no latency and high throughput. What else would you look for in such situations ? Do you happen to know/trust any professional/consultant/firm that might help solve this issue ?

-Ali