Unless you're blowing coolant out of the system, the HG is probably good.
Is the radiator 'cleaned' or 'new'?
Sounds more like a clogged radiator to me.
2 more possibilities are a slipping fan belt, and the fan clutch is slipping to much.
The fan belt could look/feel tight, but be too narrow for the v-groove and be slipping.
You could put a solid fan (no clutch) to remove the variable of a slipping fan clutch
