Capturing ILS lurches plane to the ground

I was flying my typical test flight from KBTR to KMSY. Programmed the FMC for LNAV (AWDAD6). Flight was going beautifully. With the radios tuned to KMSY ILS 10, I selected APP while still at 18 NM and 3000 ft msl and the PFD showed that LOC was caputured while G/S remained selected as ready. Immedialtely, the airplane lurched to the ground. I disconnected the A/P, but lost control of trim too and was not able to recover and had to make an emergency landing on the bayou. I am experienced with the DF 737-400, an find the 737NG flows quite similar, if not the same. Also, in the Throttle panel, the flaps handle does not move at all.In general the aircraft is a beauty and I have no doubt that once all the bugs are worked out, it will be one of my favorite panels.Enrique OsunaBR, LA