Doogie's suggestion is a good one. And it's actually really easy to check this yourself.
The most common failings are the little elbows which the nylon lines plug into. They crack up with the heat in the engine bay. One little hairline crack is enough to cause you grief as the ECU will be getting the wrong info as to what the manifold pressure really is and will incorrectly modify the air/fuel ratio accordingly.
Now there are two hoses that come off the back of the inlet manifold. One goes to the air filter, the other goes to a little black box (about the size of a tick tack box) mounted on the firewall. Another hose from the tick tack box runs down to the MAP sensor which is located in the ECU.
The tick tack box is a fuel trap. In theory it's meant to catch fuel vapour and liquid from running down the line and clogging up your MAP sensor. This is probably the second most common failing so far as all these vacuum lines are concerned.
So unplug it from the back of the manifold, unplug it from the Fuel Trap. And also unplug the line that runs from the Fuel Trap down to the ECU at both ends.
Get yourself one of those air dusters from Officeworks and blow compressed air through these tubes as well as the Fuel Trap to make sure they're all clear. If you put your thumb over one end while you give it a blast from the other, you should be able to tell if your lines have leaks in them.
(Don't be tempted to blow the air up into the MAP sensor though).
Now if it seems you're still having trouble, it might be that fuel has gone all the way down the line and clogged the inlet to the MAP sensor. In this case, the most common advice I've seen is to roll a bit of tissue and stick it into the MAP sensor inlet and leave the ECU standing over a radiator/heater for a few hours. Somehow any fuel up there will dissipate. I used a strand of wool.
