Those thermostats are notorious for having temp swings like your experiencing. Your other option is to go with a more reliable thermostat like Ranco, or to go Proportional.
Humidity wise, I'd open the top up a bit more. Is it completely sealed off?
Whats the weather like in your area right now? The humidity in my cages naturally rises when the weather outside is humid or rainy.
Yeah I cannot and will not afford a better thermomstat lol! The top is half sealed. Well i do live in florida....it has been rainy...