Thanks for your tips it is really informative. but why the same behavior is not happening in the stock Radiator.First of all if the pump is faulty, then a larger radiator will indeed cause a larger pressure difference accounting for the poorer cooling.
Also any time you play with the cooling system, radiator, hoses, whatever you can get losses somewhere that create air pockets no matter how well you think you have burped it, flushed it or added enough coolant. Sometimes it takes up to 3-4 attempts of burping and/or adding more coolant to fix it.
Consider waiting until car is cold, like first thing in the morning, get up 20min earlier, and then opening the cap to the turbo reservoir and inspecting to see the coolant level there. If it looks sparse, add some.
Also be sure to look at the reservoir next to the radiator as it should read between full and low. There are two lines, one about 1/4 from the bottom, the other 1/4 from the top: your level should be in between that. Don't necessarily add it there though, it's more efficient to avoid bubbles if you add from the turbo reservoir. Keep doing this until your level is as high as it will be. If it won't rise but the coolant still has not hit close to "full" in that reservoir next to the radiator, then add more coolant directly there.
You may have a pressure drop from the faulty pump, but it also sounds like you may have air bubbles somewhere which can interfere with temperature sensing and you get these unstable, quickly changing coolant temps. The above should be the easiest, low cost, low time first step to resolving it.
1. I drained the coolant installed the KOYO. installed the Greddy Sensor in the KOYO added new coolant in the Radiator slowly. started the car and started adding slowly in the turbo reservoir. took the car for a drive and logged the Temp. it reached as per the log 105C (221F)
2. I burped the car again for 20 mins adding more coolant. took the car for a drive and logged the Temp. I took exaclty the same route reached the same 105C (221F)
3. I drained the coolant installed the stock Radiator, move the the greddy sensor back on the hose using the greddy adapter. added same coolant in the Radiator slowly. started the car and started adding slowly in the turbo reservoir. took the car for a drive and logged the Temp max i reached. 92C (197F)