They had the same problem back in 2002 and had a safety recall. You should check with your local JD dealer about it. A new tank will run you about $50.00!

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?

First, try reconnecting the hose with fresh teflon tape wrapped around the threads. If that doesn't work, you should probably replace the entire threaded plastic assembly. While you're at it, a new riser hose wouldn't hurt.

There are a few different versions of the gasket( fat and soft,thin and soft,fat and firm,thin and firm) although you will only frind one or two at big box stores. Like so most things, experience shows if gasket is correct when tank rests on bowl with the right gap before tightening bolts. Big box gaskets are usually very soft and thicker than nessary. If over compressed,the gasket is squeezed from between coupling. Assuming that's what's happening,try shimming between tank and bowl. The shim material needs to be about 3/8ths thick -- + ,flat and about the consistancy of the rubber washer on flush valve you replaced earlier. The preferred consistancy is one which you can only slightly compress between thumb and index finger. Evenly shim all four corners and do not over tighten bolts. Just snug enough tank can only be rocked fore and aft 1/2 inch with one finger,not rock at all side to side with any less force than applied with palms pressing on top of tank. If there is more than 1/4tr inch between tank lid and wall,shim there also to prevent leaning back and rocking tank eccessivly. On the other hand if tank is tight against wall (unable to create gap with pressure of one finger) it may be cock-eyed at coupling which makes over/under tightening even more unforgiving. Idealy there is a gap between wall and tank but little or none between lid and wall, Only if you ask,I'll tell you why cussing helps.