I have a toilet that maintains a normal water level in the bowl most of the time. If a plunger has to be used to unplug the toilet, the water level is normal right after flushing but the level goes down over about 5 minutes after the flush is finished. It continues to do this for a few weeks then goes back to normal. I thought the trap was cracked but I don't see any cracks or water leaks. Do you know what causes this?

It continues to do this for a few weeks then goes back to normal. If it goes back to a normal level then it's not cracked. About the only thing that would pull water out of the bowl would be a blocked vent, Any strange sounds in your bathroom lately? Back to you, Tom