This exact thing happened to me. Run the engine, pull out of the gas, get back on the gas a little, incredible plume of white smoke.

Then it went away.
Did the engine heal itself? I hoped so. But at 25 years old I was wrong. It had a head gasket failure in progress.

An on-again, off-again white smoke condition is a head gasket. Stop and disassemble top of engine.
More run time won't make it go away. Perhaps a $60 can of head sealer will, but I don't know for sure.....

R.