Sorry couldnt be stuffed reading all posts. GQ TD42's were renowned for this. My mates does the exact same thing. 1stly, get a real gauge that gives you figures, not just a hot-cold reading. 2ndly, use Penrite HPR diesel 15 (15W-40) & nothing else. 3rdly, if your water pump was not genuine it's from china no matter what the store told you. I've gone thru 3 water pumps in 2 yrs in my GU TD42 to learn this. Also, is it losing any water? Check the quality of the seal around the radiator cap. If your rad is full alluminium, its prob chinese too & they often have poorly designed cap fittings. Also how old is your clutch? Old mate's overheating issues started shortly before his clutch went & even though he took all the same steps you have, the heating issue seemed to go away when clutch was replaced so I'm thinking minor slippage causing heat transfer & overworking engine.