A B757-200 Captain reported repeated occurrences of the autoflight system failing to select the appropriate VNAV mode after the 800 AGL thrust reduction on the DUUKE SID from SNA.

Narrative

Today; for the third time in my last 4 SNA departures; the FMC/Autopilot mis-performed in a very dangerous way. At 800 AGL; as per the SNA departure procedure; when VNAV was engaged for the noise abatement climb out; plane went into VNAV PATH instead of VNAV SPEED when the power came back to CLB2. The result was nose up 20-25 degrees and a dangerous bleed off in airspeed from the programmed V2 plus 20 of 160 knots to 135 and decreasing. Autopilot had to be disengaged and nose lowered manually to prevent stick shaker. This time; unlike the previous two occurrences we had a larger passenger load; so I've ruled out excessive climb rate due to light airplane. My current belief is that this occurrence is related to the new DUUKE departure at SNA which has been in effect for just months. Compared to the previous SID; the DUUKE first waypoint is more precisely defined; with two altitude constraints; which I theorize; may be causing the FMC to go into VNAV PATH and causing the loss of airspeed. One way to prevent this occurrence would be to disallow the use of VNAV at 800 FT on this particular departure; and use FLT LVL CHNG instead; to ensure that airspeed stayed at V2 plus 20 when the power comes back to CLB2.