I have had three 300D all 93s, the problem with your car is typical. Boost is MANAGED by numerous inputs to a computer, the quickest way to solve the problem is to find a good mechanic that can pull the codes and do the four pages of diagnostics. There are maybe half dozen pnematic relays that can cause a reduction in boost, if the computer gets a bad signal from any input it will reduce or eliminate boost all together, limp home mode.
Been there done that.
Ron