I currently have a Vanagon with a Subie 2.2 engine.

After warmup, my engine would hesitate, buck, backfire etc.

I checked and replaced a bunch or potential Subaru ignition bits only
to find out that it was the VW fuel pump was flaky and it was causing
the problem....it seemed after the motor in the fuel pump would warm
up (10-20 minutes) it's performance would degrade significantly.  Go
figure.

Symptoms were louder than usual fuel pump noise and major hesitation
when going uphill, but the engine ran fine downhill....turns out the
fuel pump couldn't provide the required amount of fuel the engine
needed to go uphill.

Check your fuel pump and/or fuel filter(s).  If you don't have a fuel
system pressure gauge, try swapping your fuel pump with a known good
one.