100 AMP BREAKER TROUBLESHOOT HELP

I am a mechanical engineer. I have a 200 amp service in my home. I have a detached garage that has a 100 amp service fed underground from my panel inside house approx. 15' away. I just recently set up a kiln in the loft of the garage for my daughter who is starting a pottery business. I wired the kiln myself and had the building inspector inspect all my work and he signed off everything as very good. I performed a test fire of the kiln with two exhaust fans going at same time. This took 7.5 hours and I had no problems. My daughter produced pieces of pottery and we loaded kiln to do bisque firing and after 2.5 hours, 100 amp breaker in house that feeds garage tripped. The breaker felt warm (not hot just warm), the wires coming in from garage were cool to touch. I waited approx. 15 min for breaker to cool even though it was not that hot, i reset breaker and after another 2.5 hours it tripped again. At this point I shut kiln down and have to research problem. The kiln draws 70 amps and fans about 5 amps. The kiln is wired as such. Kiln is direct wired to 100 amp safety switch with 90 amp fuses, safety switch is wired with #2 wire as recommended by manufacturer for about a 15' run in pvc conduit to electrical panel in garage to a 100 amp breaker. I must mention that the 100 amp breaker in the house panel is 31 years old. It use to be the main breaker in main panel to house before I upgraded my service to 200 amp. Question: Is the problem with the breaker? As I stated before service wires coming in from garage were cool to touch, breaker was warm. Tonight I am bringing home amp meter to check exactly what the amperage draw is coming in from garage. Any help is greatly appreciated.