I had the same problem, AND my car would heat on long highway runs at any hill, indicating the cooling is marginal.
My hardware solution is
3-core Triumph radiator
70? C (160? F) thermostat
electric pusher fan, always on
good radiator cap
coolant catch tank
blocking ALL holes, over and under and on sides so all incoming air is forced through
The software solution, applied periodically, is
cleaning out radiator, core, engine block and hoses with Thermo-Cure ($15-20 at Walmart)
filling cooling system with clean tap water
using a cooling additive like WaterWetter or Hyperlube Super Coolant (I'm trying this now)
I reapplied the S/W solution this week and drove for several hours yesterday ambient temp at 85? F and the coolant stayed under 90? C.
I had problems with GM long-life coolant so now am running without anti-freeze and depending instead on the corrosion protection in the Hyperlube additive. I live right on the coast in So Calif and h*ll will freeze over before this car will be in sub-zero temperatures.
While I was messing with the coolant, I also checked the chain tension and readjusted my doors. But that story is for another day.