Sounds like maybe it's a little underpowered. If you had two before and things were fine, and now with just one it can't keep up... then I'd say get another one. I have 2x 100W heaters in my 46g.
Regarding thermostats... look into either a Ranco Temperature Controller or a Johnson Controls Temperature Controller. They are digital controllers that take readings off a submersed temp probe, and either turn on/off the heaters based on the temperature reading. No cheapo bimetallic thermostat like all the heaters have. You just plug your heater into it, turn the heater's thermostat up somewhere above your normal heating zone, and let the controller do its work.