I had bought an used 05 Grand Prix about couple of months ago it had about 51,000 miles on it... it will stall occasionally when trying to start it, sometimes twice in a roll. The car also seems to have a weak start but only lasts for a couple of seconds, then after it will run fine.

Yes the mass air flow sensor is tied to the trans module and is used in the calculation of the shifting points. I would change the MAF. I have ran into GM cars with MAF sensors stalling engines. Those years were notorious for it

Have you ever cleaned the automatic idle speed control valve and threottle butterfly? It may be your problem. Try the following to clean these parts.
To correct this problem Buy a can of throttle valve cleaner (it is made by CRC) from the local auto parts store, NEVER USE CARB CLEANING SPRAY, IT DESTROYS THE ANTI-STICK TEFLON COATING, spray it into the air intake through the throttle body of the engine until you have used about half the can, you will need to hold the idle up while you do this, it will try to stall, after this shut the engine down and disconnect the Negative (-) battery cable for 5 minutes. The intake is now clean, if you continue to have the idle speed control problems you may need to seek the help of a professional.

sounds to me like you have a short in the ignition itself , it seems intermittent which would make me believe its the ignition

It sounds to me like you are having a problem with your Idle Air Control motor (IAC) Also known as AIS motor. See picture below.

This car, like most newer cars has a passive theft system. If the car sees a problem with the system when the car is running it will turn on this indicator. When this indicator is on the theft system will not arm until it sees the problem is no longer there. It probably needs a ignition switch but it should be checked out before you toss a new part in it.
I hope this helps. Please leave feedback if it did.

if it is not leaking then it is being burned by the engine, how many miles go by before you need to add oil?, if u are adding more than one quart every 1000 miles u have piston reing problems and or valve guide seal leaks. U won't see any smoke in most cases unless you are really going through the oil, like 1 quart every 300 miles.

My Pontiac Grand Prix is also a 99, it has 216,+ thousand miles on it, it runs great, idles good, I need to replace the oxygen sensor tho as it shuts off on me while driving or sometimes coming to a stop. You might try checking the fuel filter and pump as well.

the 2.7l has a sludge problem. This could cause the stalling and loss of power. There's a few fixes, the new water pump (which is part of the problem) doesn't dump coolant into the oil pan. Easy way to check is to simply drop the oil pan.

i had the same problem once before makeshure the gas lines arent coroded or kenked if that dont work try cleaning the fuel injectors.

change the idle control valve located at intake manifold.