Mac Pro hard freeze, extreme fan speed

I recently drove from California to Colorado with my mac pro in my car and upon setting up my Mac Pro, the problems began. Sometimes when I turn it on, it will "fake" turn on, never make the chime, and just sit there with fans running. If I do manage to start it up normally, it will operate at a much slower pace until it does a hard freeze. Then, the fans turn on full blast and the machine is completely unresponsive. A hard shut off it required to quell the fans. Apple Hardware Test shows no problems. What gives?? Any advice? I'm a part time music composer and producer and this is causing a lot of problems.