thermostat position

[whiteshark]

I've changed timing belt and tensioning roller after 91000 km and 8 and half year. Well the belt has no any harm but it I think elongated, because the belt at the tension side was pretty slack.
I also changed new clamps cause the cooling tubes had same leaking, and I also changed the thermostat rubber o-ring
I always take the thermostat vertically, but now (I don't know why) horizontally.
Since I have some overheating problems (90 C) maybe can't flow the coolant well?
Or I have to set ignition?
I've bleed my system 3 times.