Volvo Repair: Volvo 740 wagon 1990, fuel pressure regulator, vacuum line

Question
A few months ago our Volvo started blowing black smoke and in the middle of driving along would start to die, even if you are pressing on the gas. Took it to our mechanic he replaced the fuel filter,(said the fuel pump was fine) gave a tune up, new fuses..drove it home and it was great the next day driving along and the car tries to die..took back to mechanic and he cannot tell us what is wrong.  The car starts up and runs sometimes but not every time. We are at our wits end..Please can you tell me if you know what could be causing this problem?

Answer
  Kelly, check the vacuum line to the fuel pressure regulator when it is blowing the black smoke. Shut it off and immediately check the vacumm line to the fuel pressure regulator. Pull it off and if it's wet inside or fuel runs out. Replace the fuel prssure regulator itself. Very common.

Mark