The high speed fan is only supposed to kick in if:
1. The Dual Pressure A/C switch tells it to.
2. The computer tells it to under when engine temperature exceeds high limit - around 215-220F.
Other than that, the fan usually uses the low speed operation.
The only way to determine what is going on in your cooling system is to measure its performance with an acurate thermometer. It might be that you have a guage or sending unit that is not consistent and that your cooling system is perfectly fine.
Only way to know for sure is to check it with an external device.
Steve
