Internet Freezing-Crashing

Been w/ Comcast 5 months- plan is supposed to provide - Download of 150 mpbs - but several times a day it freeezes-crashes - and spedd test then show it at -- 8 mpbs.
Locations 3 differnt places, repairmen came to fix 4x, mocems replaced 4 times. Last repairman siad my computer was not pulling in the 5G signal, only the 2.4 signal. He directed me to buy a speicifc wifi Adaptor to pull in the 5G. --
Still having same problem.

Hired a computer man to check out my computer 3x. 
He is certain it is not my computer and that my computer is fine 
but that it is the signal from Comcast. 
Am paying for something I'm not receiving - and pricey.

My business is at a standstill because of this. 
Up north ran my business 9 years with internet providers - with no issue.

Don't know what to do. 
Does someoen knwo what is going on here?