Aside from a regular cleaning, the siphon jets should get a good cleaning at least once a year. Random vertical discoloration lines are a key indication some of the siphons are being clogged with build up.
Start by shutting off the water supply and flushing the toilet. Then dump a gallon of water into the bowl to flush the rest of the water down. Locate the siphon jets underneath the rim with a small hand mirror. Clean the holes with a hanger, stiff nylon brush, and all-purpose purpose cleaning solution.
The next maintenance step requires vinegar and duct tape. At the end of the day, when nobody else is going to use the toilet, empty the tank and seal the siphon jets with the duct tape. Then pour about a gallon of pure vinegar into the tank and flush. The vinegar will flow to the rim and be held there by the duct tape. If you can’t get the tape to stick, try using little rubber stoppers to block the siphon jets. Let it sit at least overnight, but longer is better.
Finish by removing the tape and brushing the surface again with the brush. Turn the water supply back on and let the tank refill. Immediately flush the toilet to let the water wash away any of the vinegar and loosened buildup.