Vacuum leak is possible. But sounds like its intermittent. Engine should idle high after startup all the time. Just briefly though until its warm.
What are all these mods about ? Aftermarket stuff may give you more power but it is almost ALWAYS at the expense of driveablility problems. Also the aftermarket stuff unless rediculously expensive is almost never built to the same standards as the factory components.

If a performance cam has been added or if igniton timing has been modified for race fuel this would explain the problem.

Check the ignition timing and the fuel pressure. and if the fuel filter is not new change it. I have also seen a car with an improperly installed timing belt act this way.