Short - Is that even possible? I usually think of them blowing out the diaphragm or starting to loose pressure.

Long - Car has run fine for years with the same setup (71 Challenger, street build 360, parts store fuel pump, holley 670 street avenger) No issues with fuels, no vapor lock, etc. About a moth ago I started smelling raw fuel on occasion, usually after quick acceleration to
deceleration (not racing, more like trying to make a light in traffic and having to slow down instead). I suspected a weak needle and seat.
Got home friday and it dieseled, popped through the carb, and sent a huge cloud of fuel vapor up from all sides of the hood and out the T/A scoop. So much like smoke I reached for the extinguisher thinking I had lit my air cleaner on fire. Took off the lid and it had puked easily a 1/4 cup of fuel up into the air cleaner. So, since I had been planning to switch to a street demon anyway, with a 625 version rebuilt and ready to go I went ahead and swapped it on. Thing will NOT run right at all, mostly disturbingly it is wildly inconsistent
One minute it acts starved for fuel and air, next minute its almost like an obstruction gets cleared and it takes off running fantastic, then falls on its face, dies at redlights, runs great again and then dies in my driveway with fuel vapor pouring out throat.

I'm stumped. Never had this much trouble understanding a carb issue. Vacuum leaks are admittedly a real possibility with the new carb install, and I'll probably try a new fuel pump and check for vacuum leaks with the street demon while I wait on replacement holley needle and seat assemblies in case the street avenger needs to go back on.

Really having a rough couple days in the garage this weekend.

Sounds like the street avenger had sticking needle & seat. If it worked fine before why not rebuild it? Do you have a good filter between the pump & carb?

I actually went through the street avenger about 5 years ago, but I am ordering a pair of needle/seats as a back up plan. I've had the street avenger on it for probably close to 30 years and was never 100% in love with it. Had a heck of a time fixing the bog issue common to them. Really wanted the street demon to work as it resembles a T-quad unless you look close, and I had just rebuilt it an had it ready to bolt on.

And yes, I have a factory style metal cannister filter between the pump and carb.

Only mech pump issues I've ever had was either leaking (punctured diaphragm), or weak (just plain tired).
I'm assuming it's not a higher-PSI pump than called for.
I'm thinking stuck needle & seat, and hope you've got a filter in the system.
Could also be wear, I guess.
Even with a fuel filter, junk seems able to enter, or maybe it's debris already in the carb.

No, I don't see how the mechanical fuel pump could suddenly increase its output pressure. The pressure is determined by the stiffness of the spring.
Maybe if your fuel tank vent was plugged and somehow pressure built up in the tank, that could cause it? I would recommend putting a T fitting between the fuel pump and the carburetor and connect a gauge to it to monitor the fuel pressure while the problem is occurring.

Just from my experience, semi-retired I build a few engines every year, I have a dyno and most of my engines are for muscle cars, 375 to 500 HP range most use factory mechanical fuel pumps. I have had a lot of trouble with pumps supplied by the local parts store. I always hook up a fuel pressure sensor to record on the dyno. Excessive fuel pressure at idle and low pressure above 4,000 rpm are the most common issues. Pretty easy to return when you hand the counter guy a dyno sheet and show them the numbers. I rebuilt a AFB for a guy a while back, he called complaining it was flooding, checked fuel pressure 15 lbs at idle, new over the counter stock fuel pump. I buy decent mechanical pumps from summit most of the time now. A lot of my engines are for guys restoring cars so they want a stock pump. So I just record the pressures and exchange a few.