My first guess would be bad solder joints somewhere on the board or where the display wiring meets the board. The 2nd guess would be one or more questionable electrolytic capacitors/power supply issues.
I had one of these MANY years ago and there was a chip that when into thermal runaway in the unit and it was like the squelch would just overcome the entire signal and I finally just dumped the unit. I was able to isolate the chip with freeze spray, but it was many, many years ago and I think trying to locate the chip was not worth the time and effort.
But I will say my 210 got a lot of use while it was working, this was my first programmable scanner as I recall.