'92 400SE
Today we reached warm 70 oF weather and the following began to happen:

When idling, I rev up the engine, then release the throtle, and all of a sudden the engine will drop below 600RPM and die OR come very close to stalling while it hesitates to stays alive.

When I had it in REVERSE, I pressed the gas far enough to make it go above 2000RPM, the vehicle moved a fair distance and died as the RPM's were dropping. I waited a minute, and with the help of having the throttle pressed it came back to life.

While in DRIVE, I have no problems.

Recent services (500 miles):
New sparks.
New fuel filter.
New breather hose from brake booster.
Oil Change w/ ELF 5W40 LDX.

Could this be signs of a vacuum leak...or a 100 other things.

Ideas?
Thanks.