I would get a cheap timer from walmart. I put everything on timers to get control. Have your fan kick on for a few minutes every 15 minutes or whatever length of time it takes to properly regulate the temp and still keep your humidity up. You ventilation fan may be too strong too. It should...
I would be willing to bet your problem is the distance your plants are from the light. Really common problem is not getting light close enough especially with new growers. What kind of light are you using and how far away is it???? If you are using flouresant; I try to keep the lights within...
You would have a lot better grow environment using the same space a fridge would take up by using a decent grow tent. You need the extra space from something larger for your venting and fans. It is sometimes harder than you think to maintain that optimal environment you are shooting for.