Are you opening up your greenhouse during the day? even on a cloudy day with a little sunshine now and then and quickly over heat a house. Even on a cool day with sunshine I have my houses all open. side rolled up and ends opened. If it is windy I let down the side part ways. If you don't have any openings to open you will constantly have an overheated house. Shade cloth won't "help" your plants. It makes too much shade and the plants stretch or don't produce flowers for fruit if you have fruit bearing plants started. Otherwise a vent/s and an automatic fan with a thermostat will help stabilize the temperature. I depend on mine especially in the Spring as I am starting up the greenhouses with new plants and I have to be away for the day. Good luck.
Have you heard of "aluminet" it allows the light through but reflects the heat. a bit pricey, but maybe worth it for your crops if you are doing it to save money ( in the long run on electricity or growing a crop for consumption) or sell any plants.
