95 pontaic bonneville intermittant loss of power

SOURCE: 1995 Bonneville

the problem is probably one of the battery cables. check for looseness and corrosion. especially at the positive battery connection where the two cables are stacked.

SOURCE: engine dies unexpectedly sometimes while driving

crankshaft sensor

SOURCE: Car not running on all cylinders

the first thing that comes to mind is ignition system faults. Are the spark plug wires connected properly, in the right firing order? Has work been done on the distributer, or the engine head, recently? Ignition timing can be 180 degrees out, and the engine will still run, sometimes. If it is not the ignition system, the next most likely culprit is the head gasket, which seals the head to the block of the engine. If this gasket is breached seriously, it can cause a cylinder(s) not to fire. Is there water in the oil, as evidenced by a thick, white foam? Is there oil in the coolant? Either of these are symptoms of a leaking head gasket. 
Basically, one cylinder isnt firing for one of a number of reasons. The most common cause is an igntion fault. This will cause the engine to feel and sound "lumpy", and the power would be greatly reduced. The best way to find the cylinder which is responsible is quite simple but care must be taken. Firstly if you hate electric shocks as much as I do, find a proper pair of electricians insulated pliers, and even some decent rubber gloves if you can aswell. Start the engine, and one at a time, use the pliers to remove a lead from the top of one spark plug, if the engine almost stops, or gets worse in any way, it is not that piston which isnt firing. Repeat the proccess, one at a time, you will know which piston ISNT firing when you remove the plug lead, and there is no change in the engine sound, this is your defective cylinder. Its just a case then of looking at the condition of the spark plug and asessing the cause of the misfire before carrying out repairs. BEWARE:- THE LEADS WHICH CONNECT TO THE SPARK PLUGS CARRY SEVERAL THOUSANDS OF VOLTS - HENCE RUBBER GLOVES AND INSULATED PLIERS 

SOURCE: 1997 Bonneville - Engine light code P0303

the code you have is for a misfire on #3 cylinder. (im sure you knew this right?). the crankshaft position sensor will not cause one individual cylinder to misfire. what i typicaly do in this scenario is switch the ignition wire off of the guilty cylinder onto a known good firing cylinder and see if misfire moves to the other cylinder. i repeat this process by switching the spark plug and finaly the fuel injector. a compression check may be in order as well (but not a likely cause) unless you have access to a tech 2 (gms diag computer) you will need to clear this code after each component is moved and see if you get a new code ie :p0304 if switched to #4 cylinder..other wise if you do have access to a computer that has a misfire chart in its features you can quickly identify the guilty component. as i side note the ignition coil may also be faulty, but i believe your vehicle has a 3.8liter meaning that it has a coil pack....switch the others i listed and see ow you make out....good luck!

SOURCE: 96 Grand Am SE2 2.4L starts and dies immediately

I had the SAME thing with my car!! Remove your air bag module its a grayish looking box that deploys your air bags, my air bag light was on but it was on and off. I pumped gas in my car one night and when i went to crank it my car cut right off after a split second. My mechanic took time on it but discover that it was a secrity issue. He removed the air bag module and presto it fired right up. I thought it was a fuel filter or fuel pump. It was just the air bag module, people freaked out about it once i told them what it was....it should be under the passenger seat, i believe u may have to take out the seat to get to it.