404 walkin freezer problem

I have been fighting with this freezer for a week now. I have a system that worked fine till a week ago. It is 98 degrees outside. My suction pressure is 26 psig. Liquid is 295 psig. My temp I need to maintain is 0 deg +/- 10 degrees. My box temp would get up to 22 and seemed as the expansion valve was not working properly as my superheat would never stabilize. I had a service company install a new expansion valve today and he set it at 6 degrees superheat. I am still struggling to get my box temp down to 0 degrees. it was at 6 degrees then during defrost (20 min every 5 hours) it came back up to 18 degrees. Any ideas? 0 degrees used to be easy to maintain before.