So, time to report back after some experimentation.
It seems like this tea has been ruined by spraying, at least I don't know any other explanation. I changed the water and everything and drunk quite some other teas with the same settings and there was nothing.
But with this one, each time the same thing: A numbing sensation thats especially prevalent on the tip of the tongue. Its such a shame since this tea is quite unique and I would have stocked up on it otherwise. Hadn'd thought a 2002 Gushu to be so tainted already. When I got it about a year ago, I didn't notice that, but then, I was very new to Puerh. Whats especially striking is the intensity with which this effect occurs - I think we all know the nagging feeling that something might be not right, but when its too subtle to say for sure. I had quite a number of teas which I doubted, but don't have the experience to say for sure if they're off - not so here. A few seconds after drinking the sensation sets in and after a minute or so, its really quite intense.
What a shame about this otherwise well-crafted tea, but it has been ruined for me.