After take off my aircraft takes on a pitch up attitude until eventually it stalls. Adjusting trim makes no difference as trim constantly goes back. This happens even if the autopilot is set and on all aircraft, any ideas?
Disconnect your Autopilot, trim the Aircraft for slight nose up, and continue the climb MANUALLY until you reach Cruise height....
...then, after selecting your desired Altitude and Heading, enable the Autopilot.
Always Re-trim the Aircraft after switching off the Autopilot. (When you land, the Trim Wheel may be in the wrong position for the next take-off!...take a peek at the Trim Wheel Indicator before take off!...

...!
Operating the Elevator Trim Wheel is as important as using your Yoke/Joystick..
...(Pilots often fly with one hand on the Trim Wheel during manual flight)..

...!
Paul...G-BPLF...FS 2004...FS Nav... 8-)...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.