Your parging is crumbling because of excess water coming in from the outside. This is a problem that is common in houses built with stone walls. If your property's landscaping slopes toward the house, then rain, snow, and melting snow will flow toward-and into-the house. Similarly, if your rain gutters are dropping water directly beside the house and the water has nowhere to flow but back toward the house, it too will enter the house through your foundation. A third possible problem (but not as common as the first two) is that the water table directly underneath your house may be relatively high. Again, excess water from rain and melted snow in spring will force the water table higher, and unfortunately, into your house. Once you have corrected those problems, you can tackle the basement wall.