5 problems all at once.
 

(Edited 8/27.... Problem now diagnosed. My mechanic has now found out that the vacuum pump is toast)

I was on a 12 hour drive and all went well until 8 hours into it. I took an exit off the highway and when I coasted to slow the car from 70 mph down to about 40 while coasting, it sounded like there was a small "poof!" sound like something blew, and the engine sounded a bit rougher but I turned the radio down and tried to listen but I thought maybe it was just my imagination.

Then.....

1) When I applied the brakes to stop, I no longer had power assist brakes.
2) When I accelerated it either was getting no turbo boost or maybe more accurately no kick down. The car would accelerate but didn't have any real get up and go.
3) When I stopped the car, the engine continued running. I had to use the "STOP" button on the engine to stop it.
4) When I restarted the car and drove it, there were some hard shifts.
5) When I went to lock the car doors with the key, one did not lock.

Sounds like a vac problem somehow/somewhere. I didn't do anything that would have moved any vac lines and this problem would almost have to be a tee intersection to cause this many problems all at once. I had a smoke test done about 3 years ago and they changed out any questionable lines or tees at that time though. Maybe the vacuum pump quit? There was a 12 inch spot of clear liquid under the car one day, no odor to it, that I figured was just a/c condensation but maybe it leaked out from the vac pump but I'm not sure if they have any fluid in them or not.

Any suggestions on what might be causing this?

1991 300d, 223k

First diagnose the vacuum system starting with the brake booster. You’ll need a vacuum hand pump and some golf tees. Clear liquid could be brake fluid.

If you lost the vac pump, be careful... if you have one of the earlier "ball dropper" pumps, you don't want it dropping a bearing into the timing chain.

Here's a "not too dissimilar" problem...

http://www.peachparts.com/shopforum/diesel-discussion/284094-failed-vacuum-pump.html

Sounds like a failed vac pump to me. Your '91 has the caged bearing type so little danger of engine damage

I'm not sure it does...my 91 (manufactured 8/90) has the older style. The 92 I had had the newer style.

All those problems you stated have one thing in common......Vacuum. Look for a blown or disconnected vacuum hose, or worst case scenario, a bad vacuum pump.

My '91 (manufactured 10/90) had the old style too.

Well, the lack of engine power sounds pretty scary now. Especially with some of the previous comments saying that your '91 could have the early style vacuum pump bearing. If that is the case, then it sounds like you have bearings lodged in your engine now which could have destroyed your engine. Take the vacuum pump off and post pictures.

I guess I am wrong. I thought the improved vac pump was introduced with the late W123s.

Regardless, I don't think there is engine damage ... yet. So long as the engine is still running relatively smoothly and without expensive-sounding noise, a loss of power does not indicate serious damage.

I had a similar failure on an old W116. I had it flatbedded to a shop and they inspected for damage and replaced the vac pump with no aftereffects.

The later 124's had vacuum operated waste gates on the turbo. If you lost vacuum (bad booster, bad vac pump, etc) that would explain ALL of the problems except for the door (you should have an electric vacuum pump for door locks).

Does the 300D have a different electric pump for the door locks? I thought only the 300SD had that.

124's have the electric pump. This is not a 123 in discussion.

My mechanic said it's the vacuum pump
 

My mechanic checked it today and said the vac pump is toast.

My mechanic said it's the vacuum pump
 

My mechanic checked it today and said the vac pump is toast.

Glad you are on the right road to fixing it!