Problems rarely, if ever, "fix themselves". I'd be willing to bet either a sticking check valve in the vacuum pump or a tear in the brake booster diaphragm causing the intermittent total loss of vacuum.

As said above, the shutdown vacuum pod not releasing will be in the key switch mechanism. Once the key is "started", it should release vacuum to the shutdown vacuum pod. This is a separate issue from the rest of the system.

To isolate your problem areas, pick up a hand vacuum pump and start pumping up various circuits. You'll quickly find the problem areas which you can then isolate or repair. I have one of the $12 Mity-Vac "clones" from Amazon that works great, so it doesn't need to cost a bunch of money, but can certainly save your hair and sanity!

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]