Sorry to ask the obvious but how do you quantify overheating ?
What ACTUAL problem is there, is it boiling water out the tank ?, do the fans cycle on and off as they should ?
The bit you qouted is pure shit tbh, temps are regulated by the stat when in motion above around 30mph(sustained) fans should not be needed, if they are then the fans are the least of your problems.
Of course there comes a time when there is insufficient airflow for "natural" cooling when the fans come on, if it is at 100C it doesn't matter so long as they bring the temps down so they cut out again stop worrying, 100C is nothing, and not even remotely boiling wise for an engine, mine are set to come one at 95C and go off at 92C, but in winter will be 100C and 96C, to give me a better hetaer when stuck in traffic ha ha!!
The temp gauges are notoriously unreliable/inaccurate.
You could easily convert to single speed fan if you wanted and have your own set temperature to come in.
Other things you can do is to panel in the rad, and any leak points where air can escape between rad fans etc, but I think you are worrying about nothing, you haven't said it does x/y/z when it "overheats" so I assume the only issue is the gauge reading ?
Two stage fan is for air con, the slow speed runs almost constant when in slow moving traffic to cool the condenser, the high speed is just the "normal" fan.
tabetha