I'm required to turn the blower off then on again once or twice, then the flame reappears. The pilot never goes out. The flames goes out early on and later when the unit is hot. This can go on all night, or can run fine for a couple of hours. Any suggestions anybody?

First you need to take an air compressor & clean the unit, by blowing the orfaces out very well. I would say that it would take care of the flames not stay on. It needs to be cleaned at least once a year.

I need to buy a replacement thermostat but It doesnt say anywhere which one i should buy. It just says millivolt. So should i buy a 24mv one orf the bigger one

Hello eli:

The Heat N Glo unit most likely has a standing pilot (one that is always lit). The standing pilot flame heats a thermopile millivolt generator. This current is around 200 millivolts. When the thermostat calls for heat, it closes an electrical connection allowing the 200 mV to energize the gas valve. If there is any resistance across the electrical connection, there may not be enough energy present at the gas valve to allow gas to flow.

Most standard home thermostats operate from a 24 volt (A/C) transformer current.

You will want to purchase a thermostat that has a very low resistance and is rated for millivolt usage. Read the package carefully, &/or ask a sales person.