the temperature fluctuate large (300-1500K) in a NVE ensemble

Hi, everyone:
I try to run a NVE (T=298 K) dynamics to simulate the gas diffusion. The initial structure has been optimized and run in an NPT ensemble. However, the temperature and energy can be controled. The temperature keep increaseing from 300 to 1500K in 1 ns. And the energy varies from 56000 to 65000. Could you tell me why this happens? The energy deviation are set to default vale 50000.