Certainly the collapse of both the Weimar Republic and the French 4th Republic are usually blamed on their use of PR; I remain to be convinced its the optimal solution.
It's interesting how people retcon history.
The Wiemar Republic was imposed by force by parties who wanted to keep Germany weak following Germany's defeat in the First World War. So sure, the selection of a proportional representation system is a bit damning.
But various parties (particularly, the German military and allied industrialists and nobility) were working towards the destruction of the Weimar Republic since its creation (long before Hitler became a factor). A more unified government would have just made the transition even faster.
As to the Fourth French Republic, they lost Dien Bien Phu (and as a result French Indochina) and were set to repeat that performance in Algeria. That gets blamed on the disunity from proportional representation, but I just don't see France keeping these territories no matter what government it has. They were still rebuilding after the destruction of the Second World War. First-Past-The-Post doesn't magically build up a powerful military in a few short years.