... Cotton does stretch some as you sweat and warm up so its not your imagination. Your belts and ties do stretch out and get loose.
I have a Bu Jin women's medium weight dogi, and love it. I didn't think I'd like the elastic, but it does serve as a good backup if the pants come untied. Mine do not have a zipper - just elastic and a drawstring. Cherie has a good point about cotton stretching during class. The drawsting in my pants is a nylon (?) cord, so it doesn't stretch. You could try replacing your drawstring with something like that (carefully...).
The jacket also has the modesty ties. My belt has never come completely undone, but even so the ties help keep things from getting bunched up.
I try to be in the habit of checking/tightening my belt from time to time, so it doesn't work loose.