I am in the middle of a similar problem. My 09 Tenere has done about 12000 miles, I have only had it since last winter and did one ~1200 mile trip on it a few weeks ago. Since then I; replaced the swing arm (snapped chain....), put a new rear tyre on, put 4 way adjustable lowering links on (initially at the - 25mm setting - too low so changed to - 10mm) dropped the yokes down the forks 25mm.
Loaded up and set off for a trip. The bike was practically unrideable below say 30 mph, refusing to steer at really low speeds and resisting input then diving into the corner at moderate speeds. Seemed fine above about 50.
I turned round went home and reduced the forks to - 10mm and set off again. It seemed better so off I went. The handling just got worse and worse, really slow and it would not respond to input, especially stopping, like it was on a rail, only straight on possible, a bit faster (15 to 30 say) it resisted steering input and had to be forced and would only hold a line on a bend with significant positive throttle and super gentle input. Going up to 50 more of the same really, but a bit less violent if asked to do something it didn't want. Over 50 it was fine.
So I coped with this for a 600 mile trip - luckily it was dry - I think I would have been off it in the rain. It was very weird as a natural input got a wrong response and I had to react and correct very quickly, working out what to do....

Of course, so many changes I didn't really know what was causing the problem and maybe a mix. Very early on in the ride I had checked the tyre pressures and they were OK.

So when I got home I put the forks back to normal and had a very short ride round the block and it seemed OK. However.... last night I dragged it out to clean it and it wouldn't steer just being walked backwards.....

So I got the front end in the air and finally realised that the steering was dodgy, notchy and sticking, sometimes would move OK, then sometimes stick. Head bearings then. Pretty poor to have fialed so soon. I will order some and get it sorted, then we'll see if I can get the front a bit lower again - assuming it is OK once the bearings are in.

Don't think I have ever had to replace head bearings before. Will I be able to get the old ones off and new ones on OK myself, or is it a workshop job? Anybody got any other tips, general or specific to the Tenere please?
Paul