I'm not sure what a "defined screen res" is, but if you have a poor / excessively long HDMI cable, then setting a lower screen resolution should help because a slower clock over a long cable will be more stable.
Using a shorter or better quality HDMI cable (if you have one) should help too, as well as eliminating any HDMI adapters / splitters / multiplexers, if you happen to use any.
There is also
Configures the signal strength of the HDMI interface. The minimum value is 0 and the maximum is 11.
The default value for the original Model B and A is 2. The default value for the Model B+ and all later models is 5.
If you are seeing HDMI issues (speckling, interference) then try 7. Very long HDMI cables may need up to 11, but values this high should not be used unless absolutely necessary.
Remember that if you want to boost the HDMI signal strength, you should not use
hdmi_safe=1 as it sets