A B737-800 FO TEMPORARILY LOSES CTL OF THE ACFT DURING A WILD GYRATION OF THE GS FROM FULL UP TO FULL DOWN ON A COUPLED APCH INTO IAH; TX.

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DURING THE APCH INTO IAH ON A UIO-IAH FLT IN A B737-800 ACFT; THE ACFT INITIATED A DIVE. I WAS FLYING WITH THE AUTOPLT ON. CAPT WAS PNF; AN IRO WAS OBSERVING. WX WAS VFR WITH THE ARPT IN SIGHT. WHILE TRACKING THE LOC TO RWY 27 INBOUND; WE WERE INTERCEPTING THE GS FROM BELOW; LEVEL AT 2000 FT. I NOTICED THE GS INDICATION QUICKLY (WITHIN 1-2 SECONDS) MOVE FROM THE TOP TO THE BOTTOM ON THE ADI. THE AUTOPLT IMMEDIATELY INITIATED AN AGGRESSIVE DSCNT TO CAPTURE THE FALSE INDICATION. I TURNED OFF THE AUTOPLT AND SMOOTHLY TURNED THE DSCNT INTO A CLB BACK TO 2000 FT. THE BOTTOM OF THE DSCNT WAS 1600 FT. AT THAT POINT; I MANUALLY FLEW THE LOC AND GS INDICATIONS AS THE CAPT ATTEMPTED VARIOUS MODES ON THE FLT DIRECTOR. AT 1200 FT ON THE S; THE TWR ISSUED A LOW ALT ALERT. WE SAID WE SHOWED R ON THE GS. HE SAID HE DIDN'T THINK 1200 WAS CORRECT FOR OUR POS AND ASKED IF WE HAD THE FIELD IN SIGHT. WE SAID YES. HE CLRED US FOR A VISUAL. AT THIS POINT; I WAS LOSING ALL FAITH IN OUR GUIDANCE. I RATIONALIZED THE SIT BY DECIDING THAT WE MUST BOTH HAVE THE PARALLEL RWY TUNED SO I CHANGED FREQS. THE CAPT SAID THAT WAS NOT THE CASE (WHICH IT WASN'T) AND I ASKED HIM TO RETUNE ME BACK TO THE CORRECT FREQ. THE CAPT ALSO STATED AND I AGREED THAT THE SIGHT PICTURE LOOKED CORRECT. WE MADE A NORMAL LNDG AND TAXIED TO THE GATE. AT THE GATE; I ASKED THE IRO IF HE SAW ANYTHING I MIGHT HAVE DONE TO CAUSE THE SIT AND HE ASSURED ME I HAD DONE NOTHING ABNORMAL -- IT WAS JUST ONE OF THOSE ELECTRONIC GLITCHES. WE WERE FOLLOWING A B757 AND I HAVE HEARD OF SIGNALS BEING DISRUPTED BY OTHER ACFT. AS I LOOK BACK; WE COULD HAVE DONE A GAR; BUT WE WERE VISUAL THE ENTIRE WAY AND THE GUIDANCE WASN'T REQUIRED. WE COULD HAVE CONFESSED OUR EARLIER ALTDEV CAUSED BY THE AUTOPLT TO THE CTLR; BUT WE DIDN'T AND WE WERE ALREADY BUSY. IF RWY 27 AT IAH HAD A VASI OR PAPI; MUCH OF THE MOMENTARY CONFUSION COULD HAVE BEEN AVOIDED. THE CTLR PROBABLY HAD THE ALT ALERT BASED ON THE EARLIER DEV AND JUST DELAYED THE CALL TILL WE WERE ACTUALLY R ON THE GS AT 1200 FT. LOOKING BACK; WE SHOULD HAVE CONSIDERED MAKING A LOGBOOK WRITE-UP; ALTHOUGH I DON'T THINK THE PROB COULD HAVE BEEN REPRODUCED BY MAINT.