Get an upper hose that has the spiral anti collapse spring in it or one that is wire reinforced.
Drill a 1/16" hole in the separator plate of the thermostat.
Try a new pressure cap with a higher rating.
Too much coolant expanding out of the system and not enough air or additional liquid from the recovery resevoir can get back into the system as it cools. Might be a blockage on the recovery system there.Most closed system radiators have a "fill to"line on them.The tanks on the radiator need some open space or headroom in them for the hot system liquid to expand against.
As a closed cooling system cools it will produce a partial vacuum or negative pressure ,with respect to atmospheric , in the system , thats how the coolant recovery system works.
A partial vacuum has to be present in the system to create this condition and that is normal.
The same principle is used to induce a vacuum in home canning jars.
Steam heat exchangers will do this also when they cool down. Special vacuum breaker valves have to be fitted to them so they will operate correctly and to protect the steel shell or ????? from collapsing.
An electric water pump left to run after shutdown could also possibly cause this if the stat shut tight as it cooled down and the pump was dead heading. a 1/16"Relief / bypass hole in the stat plate along with the reinforced hoses would lessen the effect of that as well.