our old year 2000 1.4 Leon did something similar although not at start up.
When driving it would just cut out and resume in an instant. Literally like someone just turned the key off and back on in an instant.
Dials would die and return to zero, all lights off along with the fright when all drive suddenly stops while your on the road with traffic behind you and then cuts back in just as quick with a jerk
Real heart in your mouth moment with fright as to "what the fukc" was that" kind of way
Never any consistency when it happened other than maybe low in the revs in any gear with a slight load on the engine
Main dealer said he had a similar
problem with the exact same 1.4 engine model (can't remember the type sorry) and had stripped and checked everything with no avail. Swopped brains and all gizmo's electrical and it still persisted and it never showed up on the diagnostics yet it appeared when driving
Don't know if that helps !!