Same problem other machines.
I have seen this problem on various makes of machines, and in most cases it can be traced back to loose/bad/intermittent connections.
Rubbed through shields, or intermittent breaks in shields to encoders can also create these problems.
Simple Explanation:
Many parts of the IO processing of a CNC machine are interrupt driven.
Normally the quantity of interrupts do not take up all the CPU time. There is still some left for normal processing of interrupts, switch scans, movement,IO queue processing, G-Code processing, display update probably in approximately that order of priority.
If any of the higher order priority tasks use up all the CPU time, then the G-Code processing (which feeds queue processing) and display updates suffer and show as a symptom.
A dirty or intermittent manual feed or speed over-ride potentiometer can also create extra interrupts by the output varying, like a noisy volume pot on your stereo. Try the pots in some other position to where they normally operate. Maybe just some contact cleaner is required. Only clean if it evident that this is producing a problem.
otherwise...
Almost certainly something(s) are making intermittent connection.
Blame external wiring first, then internal connectors, then PC card connectors followed by IC sockets.
Also look for conductive dust like contaminants around PC cards and connectors. Blow it out carefully with dry air with air blast a reasonable distance from components. CAREFUL: A good air blast can REMOVE components.
Watching this space....
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.