problem with speed after take off and autopilot "on"

in the last time a i have problem with aoutpilot. (one from two flight)I have set up correctly all for take off (no takeoff beep). If i am airbone and turning autopilot ON, i climbing too much and aircraft loose speed -- to stall. (In MCP is speed V2 + 20 added by autopilot, flaps 5, gear closed, trim O.K. autothrottle on, FD on, all systems without any warning). In FMC any changes in Climb pages. (Economy mode)I need autopilot turn off and in hand reduce climb profile.. and after probably 20 sec i can turn autopilot on.. and all is O.K.