When my sister's was playing up it wasn't quite the same symptoms but we had it checked by a sunroof specialist who found nothing, but i gave the runners a good clean out and cleaned up the contacts and that fixed it. Hers sometimes wouldn't work, or wouldn't stop closed (would go straight past into tilt and back again), so we thought it was electrical.
Perhaps like hers, yours has collected dirt/grit/crap from being parked outside and it's interfering with the contacts? That's about the only thing i can think of to suggest before taking it to an aftermarket place perhaps? Or doing it yourself, but i was a bit hesistant to go pulling roof liner etc to get to it.
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