This really sounds like some poor connections.
Check the battery terminal connections are tight and free of corrosion.
Engine fuse box is next to the battery. I would check the fuses in that area, at the front of the engine fuse box there are a lot of wires held in with bolts and high rated fuses. Check these are all tight, 10mm bolts I think.
There is a main relay in the engine fuse box. It controls the power to the engine components, but not the starter. I had intermittent issues starting, crank but no start. VCDS scan showed this up for me and I replaced, no issues since.
Next would be an output test of the alternator, check there is close to 14v at the battery when the engine is running.
If all of those tests pass, get a proper diagnostics scan.
MK4 GTI - Sold
MK5 Jetta Turbo - Sold
MK5 Jetta 2.Slow - Until it dies.
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