[GPierce]

If it has a fuse holder and fuse down by the starter pull the fuse and make sure it’s clean.
That location gets oil vapors from the crankcase vent and can cause resistance between the fuse and holder.
Or you might just use a small jumper to replace the fuse temporarily and see if it fixes it.
A friend of mine recently fixed a Model A with exactly the same symptoms by cleaning the oil vapor off the fuse and holder.
Since the coil power comes off that fuse the starter won’t be affected but voltage drop from the starter and the resistance of fuse holder combine to cause a serious voltage drop at the coil.
When you release the starter the voltage momentary rises and bang the engine starts.