Site rules prohibit giving DIY instructions.
Your techs haven't been very thorough if the problem has been going on this long. Call other service companies and ask it they can perform combustion analysis and measure static pressure. If they cannot, keep calling.
Changing parts is not a valid diagnostic procedure. And it gets expensive, especially when it doesn't fix the problem.
There is a there is a link in my sig to a map of our members. You can also try this link to find professionals who have received training above and beyond the norm:
https://www.myhomecomfort.org/