A B737 ABORTS ITS TKOF 3 DIFFERENT TIMES AT PDX; OR.

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PDX FLT ABC; XX00Z. WE WERE CLRED FOR TKOF WITH AUTOTHROTTLE ARMED AND REJECTED TKOF ENGAGED. UPON ACTIVATING THE TKOF/GAR SWITCHES; THE CONFIGN WARNING HORN SOUNDED. WE ACCOMPLISHED A RAPID CHK OF THE TKOF CRITERIA AND FOUND EVERYTHING IN ORDER. THE TKOF WAS ABORTED AT 55 KTS. WE ADVISED THE TWR AND EXITED THE RWY. THE FLAPS WERE RECYCLED AND THE INTRANSIT AND OVERHEAD DISPLAYS WERE CHKED. EVERYTHING WAS NORMAL. WE WERE THEN AGAIN CLRED FOR TKOF WITH THE AUTOTHROTTLES ARMED AND REJECTED TKOF ENGAGED. THIS TIME AS WE ARMED THE TKOF/GAR SWITCHES; NO AURAL WARNING WAS HEARD; BUT THE AUTOTHROTTLE SYS FAILED WITH THE GREEN ARMED LIGHT STILL ILLUMINATED AND THE TOGGLE SWITCH REMAINING ENGAGED IN THE ARMED POS. WE CONSIDERED MANUAL PWR SETTING; BUT IT WAS A WET RWY AND WE'D USED UP SOME AVAILABLE DISTANCE DURING THE INITIAL THROTTLE ADVANCEMENT. WE THEN ELECTED TO RETURN TO THE GATE TO OBTAIN A DPI FOR THE AUTOTHROTTLES. XX25Z; WE AGAIN PUSHED BACK AND RETURNED TO THE RWY FOR TKOF. WHILE MANUALLY ADVANCING THE THROTTLES; THE CONFIGN WARNING HORN AGAIN SOUNDED. WE EXITED THE RWY AND ELECTED TO RETURN TO THE GATE FOR A THOROUGH CONFIGN WARNING CHK.