sounds like you don't have it locked down properly. i have had to set them really funny before to get the proper level. just adjust it down more than you think and see if it works.

YOu standpipe may be higher than required. Look on the inside of the tank and you will find a "Water Level" index either printed on or embossed into the ceramic. If you already have a Fluidmaster 400A in all likelihood you do not need a new fill valve. You can simply replace the rubber diaphragm in your existing fill valve. The diaphraqms are only about $1 and a very simple change out however from your description your have already changed the fill valve so we will now move on to the next step to resolve your problem. The top of the standpipe only needs to be slightly higher than that index (abt 1/2"). Flush valves are made to be a sort of universal fit so they make the stand tall enough to meet the needs of the highest tank that the valve mam be installed in. If you happen to have a tank with a bit more length & width it does not require the full height so the standpipe can be cut down and your fill valve should fit fine.

OK, so I installed a FluidMaster 400A fill valve, and the leak stopped! But now, every 10-20 minutes, it will flush for 5 seconds - I guess this is called "ghost flushing". I didn't have this problem before. I cannot see water leaking into the toilet bowl, but have read online more than a few people saying this problem started once they installed the FluidMaster. Besides replacing the flapper, is there anything else I can do to fix this?

I didn't know that hammer arresters were code in some places. So far I have one on each of the lines for my washer. Since writing the original message I have placed one on the dishwasher, which was just as bad or worse. I may stop by the plumbing shop this weekend to get a few more to add to all three toilets. As for the pressure test, the arresters seem to take care of the bulk of the symptoms once installed. I'll still swap out the valve in the new toilet since I have a spare one. I know I have a slight leak in at least one copper line, which is burried in a wall behind kitchen cabinets. (Drips occasionally show up in crawlspace.)