Heinz, from your description of your steering feeling loose while driving straight ahead, it seems likely to be the result of play somewhere in the system.
That you have checked over everything, it seems probable that the pinion to rack adjustment (mesh) may need a fraction of a turn to provide a bit more direct feel to the steering..
PLAY can be measured by turning the steering wheel from clockwise to anti clockwise UNTIL you feel resistance in both directions. That resistance being created by the road wheels contact with the ground before they start to move even fractionally.
This PLAY on my old +8 with a 15" /400mm steering wheel, when measured on the outer edge of the steering wheel, is about 7mm in total, (left to right) if more than that on your car, perhaps that is the reason you describe the steering as "loose" caused by the the lack of pressure in the mesh between rack and pinion.. ?
The same thing can occur if the rack and pinion become worn, which tends to show up in a situation when the steering is in the straight ahead position just as you describe as on a high mileage vehicle most of the steering action is in that area of the rack... Also in that situation the free play reduces once the wheels are turned away from the straight ahead position..
This usual procedure may be complicated by the issue you describe due to possible misalignment in one of your Universal Joints when trying to feel resistance as described above...?
Hope this helps and does not add confusion..?
Good luck.
