High-profile A.I. chatbot ChatGPT performed worse on certain tasks in June than its March version, a Stanford University study found.

The study compared the performance of the chatbot, created by OpenAI, over several months at four “diverse” tasks: solving math problems, answering sensitive questions, generating software code, and visual reasoning.

Researchers found wild fluctuations—called drift—in the technology’s ability to perform certain tasks. The study looked at two versions of OpenAI’s technology over the time period: a version called GPT-3.5 and another known as GPT-4. The most notable results came from research into GPT-4’s ability to solve math problems. Over the course of the study researchers found that in March GPT-4 was able to correctly identify that the number 17077 is a prime number 97.6% of the times it was asked. But just three months later, its accuracy plummeted a lowly 2.4%. Meanwhile, the GPT-3.5 model had virtually the opposite trajectory. The March version got the answer to the same question right just 7.4% of the time—while the June version was consistently right, answering correctly 86.8% of the time.