By the sounds of things the thermostat is dimming the bulb because your probe is directly under the basking light and doesn't allow the bulb to get up to temp.
You need to move your thermostat probe away from the basking spot so that the thermostat allows the basking spot to get up to temp before it dims.
Have a look at this thread
for more info.
I don't quite understand how a 100w or 150w didn't get the basking spot up to temp without a thermostat. It should do easily, a 60/75w is normally plenty.
Can you give a pic of your setup. That'll help no end and I can explain exactly how to do it.