Volvo Repair: 97 960 overheating under load, radiator hoses, ac condenser

Question
I live in AZ, always hot or at least it seems so this time of year. Anyway, my 960 overheats under the following circumstances:
1. Drive 3000-3500 RPM for extended periods (75-80MPH)
2. On grades requiring additional power to maintain speed.
3. Cools down when slowing down
4. Cools down if heater is engaged
5. Normal "driving around town" does not pose a problem.

Here is what we have tried so far:
1. Fan - ensured fan runs both low and high speeds.
2. Fan relays - at first there was a question of whether or not the fan was kicking into high speed at the correct time.
3. Thermostat - replaced - several (in case one was bad)
4. Temperature sensor - replaced
5. Radiator & hoses - replaced
6. Head checked, valve job, head gasket replaced and timing checked.
7. Verified no bubbling occurring in reservoir - there were small bubbles prior to head gasket replacement.
8. Airflow vent housings underneath in place and secured

Now we are scratching our heads on what could be the cause of the overheating. Two ideas have been brought up, including:
1. The fan is going into high speed but is not achieving total speed per spec.
2. Some sort of blockage in the exhaust system.

Has anyone run into this or have comments/recommendations?

Thank you all in advance,

coverall


Answer
it is hard to say you have to check if you did not so far  ac control switches and sensors it can be ac not turning off on time also check blocked with dirt from inside  ac  condenser or radiator air flow .do you have high and low speed you don't need spec either you have low and high or not ? but check ac ?is the problem happening with ac ? if yes turn ac  off and see what happens ?