Oil light turns while idling at stoplight - 1978 Pontiac Bonneville

Check for loose throttle body or TPS. Or vacuum leak. Cracked or broken vacuum line.

check your mass airflow sensor,( located on the air intake line,probably close to where it hooks onto the front of the car on driver side ,should be a square box with wires plugged into it) take intake hose off and spray filiment wire with brake cleaner to clean,do not touch wire with tools or hands it will break very easily ,let dry and replace hose and see if that helps.

simple test
start the engine and let idle
if idles ok , then put you foot on the brake
if it stalls ---replace the faulty brake booster
as with all faults in a cars
have the fault codes read first to avoid unnecessary waste of time and cash

First question is - is the check engine light on ?
If so you need to start with the trouble codes in the computer.
Could be a faulty injector or coil among other things.
You should have a codes for which cylinder is not firing and other possible problems.

• Push the driver's seat as far back as possible, allowing you to lay on the floor on the driver's side.
  
• 2 Shine a flashlight at the top of the brake pedal stalk and find the stoplight switch. It will be a plastic cube either connected to the brake stalk or directly opposite the brake stalk, depending on the vehicle.
  
• 3 Depress the brake pedal and feel where the brake light switch contacts the stopping plate, a metal plate in direct line with the stoplight switch.
  
• 4 Feel for an open hole where the stoplight's plunger-style button presses. If a hole is present, you simply need to replace the rubber stopper.
  
• 5 Press and release the plunger-style button on the brake light switch and ask an assistant if the brake lights are turning off and on. If not, the switch is bad and must be replaced.
  

Stoplight Switch Stopper Replacement

• 1 Clean out any remaining debris left behind by the old stopper from the hole that the stopper goes into.
  
• 2 Press the new stopper into the hole on the metal plate until it snaps into place.
  
• 3 Shine the flashlight on the stopper and check that it sits flush against the metal plate.
  
• 4 Press and release the brake pedal and ask your assistant if the brake lights are turning off and on.
  
• 5 Replace the stoplight switch if the lights continue to stay on.
  

Stoplight Switch Replacement

• 1 Loosen and remove the bolts holding the stoplight switch in place. It could be between one and four bolts depending on the vehicle.
  
• 2 Pull the stoplight switch from its mount. Disconnect the wiring harness from the stoplight switch by pressing or pulling the locking tab and puling the harness away from the switch.
  
• 3 Plug the wiring harness into the new switch until the locking tab clicks into place.
  
• 4 Place the new switch on the mount the old switch was removed from and tighten the bolts with a ratchet and socket.
  
• 5 Press and release the brake pedal several times and have your assistant check for proper operation.
  

suspect the oil sending unit, it is mounted at the rear of the engine at the back lip of the intake manifold. I recommend that you install a manual oil gage in the the oil sending unit hole and start the eneine and get an accurate oil pressure reading before you replace the sending unit.....GOOD LUCK

throttle positioning sensor

first up.Is the oil recently changed.2nd is it the right type..3rd.Oil pressure will drop down when the engine is idling.If, when you give the engine a rev and the pressure builds straight back up,there is not a problem.Its normal.Cold oil will always give a higher pressure than hot oil.Check these few things and I hope this helps

try replacing the MAP sensor, if you have fuel injection. Your air/fuel mixture is bad.