My 5600 has started reporting erratic OATs.

On a flight yesterday from Oregon to California, the OAT on the EFIS indicated +18C, which seemed correct. Later the OAT showed -3C, then back to +17C, then -24C, then back to +17C. About ten minutes later it showed -64C, then -42C, then +18C. These indicators all occurred at the same altitude.

The 5600 is in a Glasair Sportsman (if that matters) and has reliably reported OAT for two years, until lately.

I would appreciate any suggestions for how to troubleshoot the problem.

thanks,
Bob