MightyGem,
The temp is limited to the lower of the two values, either 50 C or ISA + 39 degrees. As you climb up in altitude, the Temp limit drops. When ISA + 39 is less than 50, you then operate to the ISA + 39 limit. Thus, at 2000 ft, your limit is 50 degrees, and it reduces by 2 degrees for every thousand above that. For example, at 10,000 ft, the limit is +34 C (ISA is -5 add 39 = +34)
The reason why the max temp drops with altitude is that all the cooling loses efficiency with the air density, so that the max temp must fall to keep up with the reduced cooling capability.