External Plaster Ravelling

[SeanC]

Hi all, first time posting here - hoping to get some advice on how to sort out external plaster render that has become patchy/soft in the past few years.

The house was built in mid 80s and has cavity wall. I think what may have kickstarted the issue was power washing the plaster about 7/8 years ago. It seemed to take the skin off the plaster and my Dad also informed me that the mix would have been relatively weak when it was done in the 80s (money was tight so he scrimped a bit on cement - he laboured the plasterer).
It's almost as if the plaster has gone a bit "soft" for want of a better word. And as you can see from the two pictures, paint won't adhere properly to it anymore. It's confined to one section on one gable only. Is there any solution apart from stripping and  re-plastering which he'd be keen to avoid. 

Cheers,

[Temp]

Is there water getting into the cavity higher up? Blocked gutters? 

[SeanC]

No water getting in. It's a gable wall so there's no gutter overhead.