Sencore SC61 square wave causes DMM error

[Cyberdragon]

The internal DMM errors out with random numbers from 200 to 800 (depending on the actual frequency) if you feed in a square wave under 2KHZ and try to read frequency. I input 20HZ and it reads 200 to 400 jumping randomly. 200HZ reads 500 to 800 randomly. The wave is perfectly stable on the screen though. It does seem to read ok with a sine wave and above 2KHZ square wave.

The scope was working before, it just recently started doing this. It was intermittent at first, but now it's permanent so something failed.

I don't have the service manual, only the calibration manual. (I couldn't find any others) Any manuals or advice will be appreciated.