Engine stall on startup 337B NA?

[Jhogan0101]

Yesterday i landed and got fuel (OAT 90F, Field 1000’MSL)
Plane was sitting for about 30 mins.
I got in to fire up using primer knob instead of fuel pump.
Primer seems to work better for starts imo.
I primed 4 times (front) started up, got to about 2000 rpms then slowly stalled out.
It was similar to when you shut down engines using mixture.
This happened 3 times total and then started like normal and i was able to fly for 3 hours fine.
Sumped tanks, no water, hangared plane.
Any ideas why this would happen?