My 2000 bonneville idles rough. 3.8 liter. it even

check each wire to be at good spark plug number with the image

Run some injector cleaner through the system.

I'm not sure of engine liter size? How long since a tune-up? I always check for codes, even if lite isn't on. Some of the national brand auto stores will check codes for free, if there are any, just tell them your lite has been going on and off.
Use a gage and check proper fuel pressure. I can give you fuel pressure spec because I don't know liter size of engine? Some vehicles don't have Schrader valve on fuel rail, which makes checking fuel pressure a little awkward.
If you have electronic returnless fuel system, you have a fuel pressure sensor, hopefully on the fuel rail. The yellow wire should be for fuel pressure, engine idling, use digital multimeter on yellow wire. 3 volts = 43.75 psi. 2 volts about 20 psi less. 4 volts about 20 psi more, so you get the picture.
The basics for a rough idle, check for a leak of some kind, fuel--egr--vacuum leak.

ROUGH IDLING CAN BE CAUSE BY MANY THINGS.I AM LOOKING AT LEAKING VACUUM HOSES OR INTAKE MANIFOLD GASKET LEAK.DIRTY FUEL INJECTORS AND WORN ENGINE VALVES WILL CAUSE ROUGH IDLING.ALSO LOW IDLE SPEED CAUSE ENGINE TO RUN ROUGH.

make sure you put the right plugs in the fault is most likely the coil pack follow the plug leads from the spark plugs and you will reach the coil pack about £60

at what? depending on engine temp and rpm, normal oil psi should read 30-40 psi at idle and at cruising speed of 1500 to 2000 rpm should read 60-70 psi. most bonnevilles from 1992 to the last year run of 2005 have the 3.8 liter V6 which was updated over the years.

Mechanic replaced the MAS. Car still idles a little rough and battery needle gage still fluctuates.

you need to get it scaned.auto zone will do it for free but most of the time it a o2 sensor or coolant sensor

Best thing is to get the truck scanned for any DTC, Auto Zone scans for free and can help you pin point the cause of your miss fire. It could be running lean due to dirty injectors, so injector cleaner in the gas tank may help after a few days of running the cleaner through the system. It may just be time for a tune up, spark plugs and wire change ect. Could even be a dirty Air mass flow sensor that could be causing the miss fire. 
Have it scanned to save you some time and money and most garage now will scan for around $15.00 now. Always call around for the best price when getting you car or truck scanned, most places have the scanner and would want you business if you decide to have them work on it.
Good luck and hope this helps.

The intake gaskets leaking would cause the engine to have a misfire. Get a can of Brake Clean and spray in around the intake manifold on the top of the engine while the engine is running. The engine idle will go up or down if there is a leak. Also try swapping the spark plug with another spark plug to see if the misfire moves to another cylinder. If it moves then you have a bad spark plug. I hope this helps.