about Triple-Speed Ethernet core stability in high temperature situation

Hi, I'm using Triple-Speed Ethernet(TSE) MAC core in my peoject, but I found this core seems unstable in high temperature, in my test the enviroment temperature is about 50 centi-grade, while the FPGA temperature is about 56 centi-grade, in such a situation the TSE core will not work properly, which cannot receive data, I tried to capture ff_rx_dval, but capture nothing; if I lower the teperature to 45 centi-grade, then the core can work again. I checked the timing report, there is no timing failing. It's quite strange if I didn't re-start the board the core work well all along, even in high temperature, but once I re-start the board, the MAC core cannot receive data any more. My FPGA is CycloneV 5CEFA5F23I7, 60 centi-grade should be safe. So what could be the reason?