The rads not dumping enuff heat.
If you use a temp gun on the rad inlet and outlet when it's"overheating",
The difference between the upper and lower hose coolant temps needs to be min. 40*,. 50*+ would be better.
Shoot temps across the front of the rad, every inch, halfway down, you'll find cool/cold sections. Those are plugged tubes, the larger the cool area, the larger your problem.
If the rads only dumping 10*, that's about enuff to cool a 2 cyl lawnmower.
Do you have a spring in your lower hose, to stop it sucking closed at speed ?
Doubt me, go check the in/out temps on your daily driver.
