autopilot problem

[Craig.]

I am having a problem with my autopilot. Any jet I seem to use, the autopilot will climb for 5000 feet maybe 10,000 at the set rate, however it then slows upto 500fpm regardless of the rate set. I move it up or down the plane will start to climb a little faster but then go back to 500fpm.
Any ideas?
Thanks

[DeepSpace]

Seems to happen to me too from time to time.. All you need to do is to reactivate the ALT hold after you change the vertical speed.