I'm not familiar with what materials PSE used on the Novas, but some continue to stretch until they break. I'll be willing to bet a dollar the string/cable is the culprit.

Sounds like it's time to find a new shop If this is a bow you plan to keep for awhile, you might consider purchasing a quality set like those from Winner's Choice, Taylor Made, Berry Strings, etc. Typically, once they set, quickly, they're set.