B737-700
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Undershoot
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft
Had the aircraft level at 7;150 FT instead of 7;000 FT on descent. Aircraft was in VNAV.  All altimeters were correct with no splits.  I would think that this is just a glitch; but this is the third time I've had it happen in different aircraft over the last three weeks.  It happens in VNAV mode only and errors have been between 140 FT and 400 FT every time.  I don't know if there has been some sort of software update recently but again it has happened repeatedly with different first officers so I don't think that it is just me screwing up the FMC.
The reporter stated that he did not realize until the third occurrence of the early level off that this anomaly was aircraft related and not a crew error. He is now vigilant during every approach to a level off. In order to correct the early level off; the aircraft is put in a Vertical Speed Mode (VS) descent and it then levels at MCP selected altitude.
A B737-700 Captain noted that a series of aircraft autopilots level off between 150 FT and 400 FT early during descents in VNAV.