Question: Unlike reptiles or some solid fish we as human race are very sensitive to temperature. A small difference in our body temperature, of only 1 single Celsius degrees causes fever, 4 degrees and we start to lose our mental abilities, more and we are dead. 

While on the other hand, cold blooded animals can't produce their own heat and are too dependent on their environment, plus both parasitic and beneficial bacteria have a harder time surviving inside cold blooded organisms.

So, what changes does human biology need to allow a person's survival and intelligence with cold blood? 

What could force humanity to evolve into cold blood animals?

Answer: 
Cold blooded animals have several sets of enzymes that maintain bodily functions at different temperatures, while warm-blooded only need and have one set. This would be the first big barrier to evolving (back) to being cold blooded. Not having a high enough metabolism to sustain the brain at our intelligence levels would be the second barrier.

I don't think there are any circumstances that could force modern humans to evolve to being cold blooded. We're simply too good at changing our environment to suit our needs. With stone age or earlier humans, you might still have strong enough environmental pressure to direct evolution.

Some mammals hibernate during winter, greatly lowering their metabolism and conserving energy in times of food shortages. That is probably the best route towards evolving cold-bloodedness. Humans might still have some of those genes and they could lead to a lethargic state of semi-hibernation, which over a million years or so could be refined into a mostly functional state regardless of body temperature. The catch is that energy efficiency implies lower brain functions, making these humans dumber again.

Btw, cold-blooded animals also die at 40 C or higher (vital proteins start breaking down at 42-44 C for everyone) and freeze at 0 C (or a little below for marine animals with anti-freeze, as commented), so they have a narrower temperature range than warm blooded animals.
To achieve cold bloodedness in the first place, I would suggest the removal of the hypothalamus, though this comes with its own problems.

<https://www.quora.com/What-does-the-hypothalamus-do-What-happens-if-it-is-removed>

It may be necessary to do this to babies before they are born, as this artical suggests the pancreas develops with the hypothalamus in the womb, which means it may be too late by the time the child is born.

<http://www.ncbi.nlm.nih.gov/pubmed/7019801>

It may be possible to eliminate the symptoms of missing a hypothalamus with a specific form of HRT (hormone replacement therapy) to replace the missing hormones at set intervals or as and when required. You would also require insulin injections in order to properly process sugars.

Now that you have no way to regulate your own body temperature I would suggest moving somewhere warm and keeping wrapped up. Also live near water in case you find you are too hot and need to cool yourself. Also a change in diet would probably be beneficial to aid your self inflicted diabetes.

To make the changes more permanent, a selective breeding program between  survivors of your barbaric experiments  members of your new community to see if you can naturally reduce the effects of the hypothalamus on bodyheat over the generations.