Does the car with the Proform carb have a mechanical pump?
Does it have a gauge(or can you put one on to check)?

If so........ does the needle do the same thing on that car?

Imo, the large/rapid pressure swings point to a faulty pump.
The net is full of stories like this, and many times after the problem has been solved(new fuel pump), the fluctuations are either totally gone, or are minimal.

If you install the gauge on the other car, and it reads steady....... then I guess it’ll come down to how bad you want to find the source of the issue.
You could pull the “known good” pump off that car, install it on the problem car and see what happens.
You could also install the new pump on the other car with the Proform carb and see if you have the same symptoms.