White bubbles in radiator when pressing accelerator pedal

Recently, my car starting overheating on high speeds. Sometimes it does overheat in traffic jams as well but normally it overheats on high speeds like 80km/hr +. And starts cooling down while in traffic jam. This is extremely strange behavior.

So I performed a radiator flush to flush all the old coolant which was around 2 years old. After flushing, when I started to bleed the air out of system, I opened the radiator cap and started pressing accelerator pedal. Upon pressing the pedal, I noticed whitish milky type bubbles appearing on top water surface but then they disappear when I release the gas pedal. Again when I pressed the pedal, white bubbles appeared on water surface which goes away upon releasing the pedal.