Maybe it's time to consider a more permanent repair than "parging." If it were my basement, I'd first consider doing any (and every) thing possible to keep the water away from the house's foundation, including regrading the yard if necessary, extending rain gutter downspouts, etc. If cost is no concern and you've already done those things, then you might give serious thought to doing a local search for an established basement water-proofer. If that doesn't work, another option would be to pay a good concrete contractor to excavate, form and pour a bonded concrete wall on the outside of your field stone foundation. A thickness of 6" would enable a heavy mesh reinforcement, to minimize the effects of any random cracks that might occur. Sealing the new concrete outside with bitumen or the fancier plastic bubble stuff, and backfilling with pea gravel, would be the finishing step.
Get a few quotes, and make sure whoever you select to do any work removes all dirt and loose mortar (water-blasting works well) before setting his forms and placing concrete, or installing any other kind of sealing system.