I think there's more to this - and it hasn't been discussed yet here...

First off, my main car (testbed, laboratory, etc.) is a diesel 126, so don't know how much of this applies to your 124. Given that caveat, here goes:

My vacuum pump has never failed, but I understand it may go like this if it ever does: As they start to go, they are known to start dumping engine oil into the brake boost line (the big plastic pipe between the pump and booster body). Because this line has ports which supply vacuum to the shutoff system, EGR, climate control, etc, any number of symptoms may result as this oil creeps into vacuum lines. It isn't hard to imagine that one of these might be an intermittent loss of brake boost. A very common effect is loss of the vacuum shutoff function.

In my case, a small occlusion of 'crud' in one of these ports completely blocked the vacuum shutoff circuit. After cleaning the port, problem solved!

Before getting too crazy with replacing everything: spend 15 minutes, remove this vacuum line, and be sure it's clear.

Lemme know! Lou