EGA display converting to VGA - screen's shaking
The GBS8200 upscaler chip runs on 3.3V and doesn't like 5V TTL inputs. Typical symptoms of excessive input voltage are vertical shaking and intermittent freezing of the display.    

The GBS8200's input circuitry looks like this:-







simulate this circuit &ndash; Schematic created using CircuitLab

The 1k resistor in series with the Vertical Sync input may or may not be enough to limit injected current and drop signal voltage to a safe value. You could try adding another 4.7k&ohm; in series to get the upscaler's input below 3.3V.

The horizontal/composite sync input has no series resistance so the full input voltage is applied. I have found that adding 680&ohm; in series was sufficient to get it working properly. In combination with R34 this should divide a 5V input down to ~3V.