Did you try and obtain at stat one with a lower initial opening temperature than the original Neil albeit with OE fully open temp
. If not your local motor factor should be able to help, they are available with greater hysteresis. Failing that try say an 80°C stat. FWIW this was the solution offered by SiFab to cure same issue on 4/4's fitted with their aluminium replacement rad.
Your measurements suggest the OE one was not opening soon enough given the thermal dynamics. I fear that if the new 85°C stat matches OE you will see little or no improvement to the overheating issue.
