Hello,
Indeed, it is most likely an issue caused by a failing capacitor. The Baseband Tray handles the distorsion meter function (as well as other functions, which you might see fail soon) and it was plagued by a batch of bad capacitors. Just take the tray out and replace all of them, about 50 pcs. I also did the Digital, Analyzer and Generator trays on my IFR COM120B.
Let me know if you need documentation, I have the manuals & schematics.
Cheers,
Razvan