Limp mode, normal speed/rpm then no fuel when pressing on the accelerator? Change the Thermostat (Ford Focus and others)

Ugh! I couldn't wrap my head around this one. I would have an occasional stumble, then the car would suddenly be in idle mode - it wouldn't matter how hard I would press on the accelerator.
I would get no codes. If I floored it repeatedly, it would stumble somewhat and the fuel mixture would be rich (as per the OBDII readout).


This is what I tried :

• Fuel Pump : I got access by cutting the sheet metal under the back seat. Took out the pump with the sending unit, tested the pump, cleaned the contacts, and put it back in. It seemed fine, for a bit - but the stumble continued.
• DPFE : I tried cleaning it, then replacing it with another one. No change. My EGR is so rusted that I should have tried replacing that too, but time was short. 
• Oxygen sensors  : They measure the oxygen content before (upstream) and after (downstream) the catalytic converter. Running rich is not a good thing for neither the O2 sensor nor the catalytic converter. Again, time to get a replacement was an issue. I cleaned the upstream one with MAF cleaner and put it back in. The downstream sensor was so stuck in place, I couldn't get it out.

When the vehicle became so unreliable that I felt is was unsafe to turn with the fear of the engine no longer providing fuel to accelerate, I noticed that the temperature reading on my cluster was just left of centre. Connecting my ELM327 to my Android phone and using Torque, I saw the the coolant temperature of the vehicle never stayed above 80C! When the car somehow started operating normally, it was above 83C. Should I turn heat into the cab, by allowing coolant to go through the heat exchanger, the engine would suddenly stumble again as its temperature would drop below 80C. Thermostat maintenance was required. I should have changed it when I changed my water pump last year. The thermostat is designed to open at 88C or 192F. It appears that the thermostat was getting stuck open! Three bolts, well one was so rusted I had to break the thermostat housing, three hoses and and an electrical connection to the temperature sensor, and I got this mysterious automobile problem solved.

Since I've replaced the thermostat, the coolant temperature is between 90C to 93C and my fuel economy is back up to 6.1-7l/100km or 38 to 33 MPG.