You have the Maximus VI Hero. I do too.
I have a 4770k OC'd to 4.6 GHz@1.30V and I get around 70 degs at full load at an ambient of 20 C.
The fact that someone goes to work at a tech store, calls themselves a techie and gets to play with ppl's computers doesn't automatically make him good at the job, knowledgeable or professional.
I have had techies screw up my PC back in the day including the very thing you are experiencing, bad CPU-Cooler seating and too much TIM. My system only started working perfectly and error-free only after I took the time to build it up myself from scratch.
I would advise you to stop paying bad techies money and take time to reseat the cooler yourself and reapply TIM. Take a weekend off, watch some youtube tutorials and do the job yourself.
As to why your PC bluescreens when at stock, well the M6H mobo is very complicated and there might be some setting that you have tampered with that you forgot to reset and thus your CPU produces Stop 124 bluescreens due to insufficient voltage.
Another explanation is that your 4790k has simply degraded due to electromigration as a result of running over 1.30V Vcore at temps at or near 80 C for many years.
Note that as I have found out after 6 years owning this specific mobo (M6H) it automatically increases voltage under ADAPTIVE up to 0.06 V to what you have set in the BIOS. Thus for an OC with 1.30 Vcore set in the BIOS, you will get spikes up to 1.36V.
As HARDOCP noted in their M6H-4770K review:
"I WAS ABLE TO PUSH A 4770K TO 4.8 GHZ @ 1.35V. GIVEN THE CPU DEGRADATION WE HAVE SEEN THOUGH, IT SEEMS TO US THAT VCORE VOLTAGES THAT HIGH ARE NOT SOMETHING TO BE USED DAILY UNDER LARGE WORKLOADS IF YOU CARE ABOUT THE HEALTH OF YOUR HASWELL CPU".
If your CPU isn't stable at stock frequencies at stock voltages then it has degraded, and thus you need to increase VCORE for it to be stable. If it won't be stable then it's simply time to buy a new modern CPU/mobo.