Hi oldman -- if you check Bigfish's
Large File Games you'll recognize some adventure games -- there might be a few you haven't tried yet.
In terms of third person adventures, you might consider Emerald City Confidential -- it's from Wadjet Eye (the developer of the Blackwell adventure series). The puzzles are on the easy side (I know you like tough puzzles

), but the story is excellent.
It's not in third person, but I'd also recommend Drawn: The Painted Tower. The puzzles are tougher, and the environments are unusual.
If you'd like to sink your teeth into tough puzzles and don't mind a bit of hidden object searching, try The Fall: Separation, which is by Kheops (developers of Return to Mysterious Island, etc.) --- also a first person game.