I want to throw in that it might be the idle air valve located by the egr valve. When this acts up it can cause the idle to drop to low causing some back firing, rugh idle and low vacume needed to run the brake booster, also will cause the oil light to come on if rpms get too low. On my 2001 it only does it when it is warmed up and you really notice it when taking foot out of gas peddle.