I am a home owner with a 16 yr old Lennox 5 ton rooftop combo heater/AC model GCS16R-651-75-3P. I am dealing with a loss of cooling after about 45 minutes of running. Three separate visits by professional A/C service have resulted in doing the following:

1) replaced contactor for compressor and one discolored terminal
2) replaced compressor capacitor
3) replaced evap fan motor and cap

After each visit, the unit was running well with a 20+ degree difference in return air and exit air in the house. After 35 to 50 minutes, the cool air stopped although the evap fan and the house blower continued to run .

Each time the service men came out they checked freon pressures, amp draw on the compressor, resistance on the compressor motor leads etc. and said all was OK.

The service men do not seem to have any strong leads on what to test next and its getting a bit expensive.

Does anyone have any suggestions for further testing?
Does any one know what temperature I might find on the external shell of the compressor that would cause the thermal switches in the compressor to trip? I would like to know if it is getting hot enough to trip or if it is below where one would expect the thermal protection to kick in.

Appreciate any help or ideas ...