I have a 2001 discovery SE. About 2 years ago we had the heads shaved and all the gaskets replaced. 2 months ago the top hose blew. I had it replaced. 2 days ago it began to overheat and it blew out all the coolant. I had it filled and checked for leaks. there were no leaks. Then I replaced the T-stat. It was fine for 2 days. While sitting in idle I noticed the heat did not work. Now it's overheating again. Any advice? It has about 90,000 miles. The mechanic said that the water pump is fine.

 

Get it pressure tested and a chemical block test which will tell you whether you have combustion gases in your coolant or not. If your engine has overheated in the past, you may have a slipped liner or cracked block.

Do you have the "waterfall" noise coming from your glove box or an overly-pressurized coolant reservoir? You can also pull your spark plugs and look for one that looks to be "steam cleaned". If that's the case it's not going to be pretty for you. I just went through the same thing with my 04 DII.