For someone who does not know much about cars, you know your car well enough to put out numerous and very good clues to me! Your experiences are good training and that, you should not just kiss off as you are not knowledgable!

The part about changing first to second shift means to me you are at one time or another use may your headlights. Some people alway use headlights when driving.
Depending on what the companies call first shift. To me its a graveyard shift or the first one of the day. A clue, you see, you felt relevant!

Using headlights or the A/C includes the fan will pull down the battery if there are numerous short term travels. You mentioned an inverter, what does it power? A can or lunch cooler? Does it have a low voltage light indicator or meter on it? A good clue!

I assume you do not have a voltmeter built into you dash, shame on that. It could give you a "constant readout" on your charging systems health. Electricity is just as important as gasoline!

A cheap, $4.00 hand held voltmeter from Harbor Freight will help give you a clue to what might be a little on the south side of good when hook to the battery or in most places anywhere in the car. You want to see 13.5 DC volts to 14.2 V most anytime or load usage.
If not between those two reading there is a problem.

Having an engine acting like it choking can easily be fuel pumps cutting off & on. Sometime it starts and sometimes it does not! Another good clue!
Do have any idea how old the pumps are or if the mechanic check and clean the connectors?

Has your mechanic taken the fuses out of their clips and cleaned the fuse tips. I usually hold them between my finger tips and scrub them against my denim jeans.
I clean the springs clips with a gritty fingernail filing board and coat with a oily spray. LPS 1 is my favorite but there are all kinds of concoctions that will work. Engine oil if nothing else!
This is something you could check yourself. I do not know where they are on a 900 series. On 240's they are in the driver door jam.

As you can tell I am like others it appears electrical. Your mechanic is suspicious too but sometime it can be the tiniest thing. Being intermittent its corrosion or age and wear that you can get on any car out of warranty!
Darn cars you first pay a salesman, a repairman or a parts man.

I drive older cars because I hate the first two for getting over paid.
The parts man, I beat him up at junkyards.
My labor and time to read on how things work, is money earn. It is closely related to Chinese wages dollars wise though but my savings of cutting out the others has paid for houses and my home shop over the years.
Did forget to mention the bankers who have more arms to get into your wallet than an octopus! (:-)

All my real estate has not depreciated either! My cars hold their on but vintage is, a matter of tastes, like wines!
Post back as there are others like me that live to fight these economic battles!
Phil