It could be caused by a few things, here are the most common causes...
Refurbished phones tend to have sub par parts and the CPU may be crashing under over clock loads.
Occasionally a glitch may occur when flashing a time and random reboots can be a symptom.
When downloading a rom always check the downloaded file sizes against what the upload site says it should be...
Try redownloading and flashing the rom... also, ditch no frills CPU and get a program that let's you set time based profiles.
Set profiles for underclocking, standard speed and overclock settings to see if your CPU is up to par.