I have a Nuc5i7 although it runs very hot around 50-90c when room ambient temperature is AC controlled at 24c~, I haven't experienced any show stopper moments. I did put a cheap fan underneath but its more to have the lower part on nuc more exposed than to blow air on it. I would suggest download Intel running utility and see if the processor is being correctly throttled, because the processor shouldn't get so hot that your aren't able to boot.
*Intel tuning utility
Please respond and let us know what fan speed control settings you have tried and what kinds of temperatures you are seeing during operation (especially at the point where something is going wrong). If you are at the point where it is unable to boot, jump into Visual BIOS and see what the temperature readings are there. For Windows run-time environments, there are a plethora of tools out there that will display the temperatures from the processor and other monitoring points within the system. My favorite is FinalWire's AIDA64 (http://www.aida64.com/), since you can also use it to place a load on the system and see what happens. Others you could try are Alfredo's SpeedFan (http://www.almico.com/speedfan.php), CPUID's HW Monitor or PC Wizard (http://www.cpuid.com/), REALiX's HWiNFO32/64 (http://www.hwinfo.com/), PassMark's BurnInTest (http://www.passmark.com/products/bit.htm), etc. and etc.
Scott, I appreciate you taking the time to respond. I've been peeking at CPU/system etc. temperatures in detail with SpeedFan as you suggested and must conclude that I'm probably wrong about this being caused by overheating. Thanks for questioning it!
Unfortunately, I'm realising this because the problems with WiFi connectivity loss are very frequent even when the system is cool and doing nothing.
I haven't seen the other problems the past week or so, but also haven't spent much time with NUC (since the WiFi issue makes it mostly useless to me; it usually disconnects within 30 mins of booting, often within minutes).
I've updated BIOS & drivers - maybe I received a lemon. It's going back to Intel.
I am afraid that you are having those issues with your NUC, and it seems that you have covered what we recommend to resolve the issue and since the WiFi card comes with the product from factory you can definitely get your product replaced. Please use any of the contact options that appear on the link below:
Hopefully the issue will be resolved with the replacement. If you have more questions please let us know.