MCP389 and Altitude problems

Hi,

in my 2 last flights i noticed some problems with the autopilot and
altitude.

With MCP ver389, when i'm in descent and i have an alitude ie 12.000 in MCP
and descending with Vnav when the aircraft approach 12000 cannot hold this
altitude but loose more feet, then trying to climb for return to 12000 but
overshoot them and re-try to loose feet etc. In all these phases around
12000 feet also the mode changes from descento to climb and the speeds also
are changing.

Does anyone noticed something similiar?

thanks

Eddie Armaos