Can't maintain toilet bowl water level

The water level in the bottom of my toilet bowl slowly drains out until it is empty. It takes about 5 minutes to completely drain.

The toilet works fine otherwise.

I have removed the toilet from the sewer line and filled the bowl with water and it still drained (thought my problem might have been vent related).

The guy at Lowe's told me I probably have a small crack in the porcelan bowl, but I inspected it and can find nothing wrong.

Any ideas?