B737. ACFT FMC AUTOMATION DID NOT DO WHAT THE CAPT EXPECTED.

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WE WERE IN CRUISE AT FL260 WITH A PLT'S DISCRETION DSCNT TO FL240; ASSIGNED 280 KTS. THE CAPT WAS FLYING WITH VNAV AND LNAV ENGAGED. ZMP TOLD US TO MAINTAIN 300 KTS. THE CAPT DESELECTED VNAV AND SET 300 KTS IN THE AIRSPD WINDOW. THE THROTTLES ADVANCED AND HE ASSUMED HE WAS ACCELERATING TO 300 KTS IN ALT HOLD. HOWEVER; WHEN VNAV IS DESELECTED AND THE ALT IN THE MCP WINDOW IS NOT THE SAME AS ON THE CRUISE PAGE (BECAUSE WE HAD A PLT'S DISCRETION TO FL240) OF THE FMC; THE AUTOPLT REVERTS TO CTL WHEEL STEERING PITCH. WHEN THE THROTTLES ADVANCED; THE AIRPLANE BEGAN A SLOW PITCH UP AND CLB. WE CLBED APPROX 700 FT BEFORE THE CAPT BEGAN A DSCNT TO FL240. THIS GOES BACK TO SOMEONE HAS TO BE FLYING THE AIRPLANE. THERE WERE SIGNIFICANT NOTAMS AT MSP INTL ARPT DUE TO RWY CONSTRUCTION; AND I WAS REVIEWING THEM AND REFERRING TO THE ARPT DIAGRAM. THE CAPT WAS REVIEWING THE ARR. THE AIRPLANE DID EXACTLY WHAT HE TOLD IT TO DO; BUT NOT WHAT HE WAS EXPECTING IT TO DO. WE LEARNED THAT AFTER YOU PUSH BUTTONS; MAKE SURE THE AIRPLANE IS DOING WHAT YOU INTEND IT TO DO. THIS WAS A VERY UNIQUE SIT; BECAUSE YOU WILL NOT GET AN ALTDEV LIGHT OR HORN BECAUSE THE MODE CTL PANEL ALT IS SET LOWER AND YOU ARE CLBING. SUPPLEMENTAL INFO FROM ACN 407474: 4TH DAY OF 4 DAY TRIP; SLEPT POORLY THE NIGHT BEFORE. ZMP OR APCH CHANGED STARS AND RWYS 60 MI E OF ARPT. TOLD 'PLT DISCRETION DSND TO MAINTAIN 24000 FT.' ALT WINDOW WAS SET TO 24000 FT. FMC PITCH MODE IN ALT HOLD. THEN TOLD BY ATC TO INCREASE SPD TO 300 KTS OR GREATER. I HIT VNAV BUTTON TO OPEN WINDOW OF IAS/MACH AND SELECTED 310 KTS. WE WERE 280 KTS. AFTER TOUCHING THAT VNAV BUTTON ADI-FMC PITCH WENT BLANK AND 'CTL WHEEL STEERING PITCH' ACTIVATED. BECAUSE OF THE THRUST INCREASING; NOSE STARTED CLBING UNBEKNOWNST TO ME UNTIL 26700 FT. I DSNDED BACK DOWN. WE WERE BOTH BUSY GETTING OUT MAPS; CHARTS; ETC; AND NEVER NOTICED THE CLB OR CTL WHEEL STEERING PITCH. 'CLASSIC OVER DEPENDENCE ON AUTOMATION' DOING WHAT I EXPECTED BUT NOT FAMILIAR WITH THE COMPLEXITY OF MY REQUEST.