You can rule out radiator fan problem kung nago-overheat kapag mabilis ang takbo. The fans don't normally engage at high speeds since mabilis na ang dating ng hangin - enough to cool the radiator without the fan. In your case (7K) mayroong clutch ang fan and it disengages the fan at high vehicle speeds.

I would first check the radiator. Baka nga barado and not enough coolant is flowing through the radiator at high speeds. Specially since you are using deep well water. Suggestion lang, use distilled/purified water in order to avoid clogged radiators.

Since 7k iyan, check mo na rin ang water pump. Madali na iyan tanggalin specially if the radiator is already out (hindi na masikip). Inspect and replace if necessary.

And inspect the fan clutch while you are at it. Add silicone oil if needed. Mura lang ang 1-tube ng silicone oil. But you might need 2-3 tubes.

Ayan, dumami na tuloy ang gagawin.

And if the above doesn't work, mas marami ka na talagang gagawin.