get the whole intake system pressure tested, a good test is to start car hold revs abour 2000rpm then get a can of WD40 and spray it around the area you suspect the leak you can sometimes see where the fluid id being sucked in and you also find that the revs will increase or die in some cases.

The other thing i'd check is the Map Sensor, Phase Sensor Gapping (and conditon), Crank Sensor condition and gapping also the condition of the whole ignition system