Autopilot bugging out

[Lenny Jobling]

When I set autopilot after takeoff it doesn't work. It doesn't climb up to the set altitude, the speed I set doesn't work. Also, suddenly after 30 seconds of me setting the autopilot, it disconnects and uncontrollably climbs with a vertical speed of 15000fpm. I have flown airbus, Boeing, ember aircraft on msfs and know how autopilot works. But I cannot understand why it does this. If anyone has any advice please let me know.