Thanks Jacob for responding with your thoughts. The large size of the memory allocation is what took us by surprise as well.
We have tried deactivating the ACF & Osetin plugins as well as switching themes and found that the site was stable thereafter. But when we contacted the development team behind the theme/plugins they were baffled as well. The theme is being used by over a 1000 customers and none have reported this issue. Hence it seems to be a conflict between the theme/plugin and the WordPress installation or server settings. We are considering switching to a different host.