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Treat with a detergent like dawn and backflush radiator of possible grease oil.
Do the detergent 1st, Then a treatment or 2 of thermocure, rust 911, or evaporust - Back flush through the radiator. Detergent 1st, these products are poor at flushing grease/oil. then before running add an upper hose coolant filter like a Gano. Do not want anything that breaks loose clogging up the radiator.
If still an issue try shooting the radiator tubes with a IR temp gun, if you find cooler/cold spots, tubes are partially or totally plugged.
If still overheating/no improvement have radiator rodded out at a radiator shop.
Flow testing only works on radiators that are very bad, And sometimes testing at radiator shops can be false positive/thinking the radiator is ok.
A radiator can flow ok, but poor fin contact with the tubes due to age decreases a radiators ability to cool sufficiently.
If your radiator is really old, may be time for a new one. or a core replacement.
When I back flush prefer to back flush into a large plastic tub to see what comes out. The above products typically kinda dissolves rust scale, but you may see a few flakes and some grit.
If you research more and use white vinegar, be aware it can remove the grease in the original design type water pump causing the solution to leak and get all over. After the last treatment of white vinegar, do a treatment of baking soda/water to neutralize the acidic content of the white vinegar.
Simple flushing/backflushing with water is not good enough. This is similar to if you splash/spill battery acid, just a weaker concentration