MD80 FLT CREW; HAND FLYING THE NELYN RNAV SID FROM DFW; FAILED TO FOLLOW LATERAL TRACK PRECISELY WHEN AUTOTHROTTLES FAIL TO MAINTAIN REQUIRED IAS.

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DURING TAXI OUT; FO CONFIRMED NEYLN TWO RNAV DEP WITH QUEUE. EVERYTHING WAS PERFECT. DEPARTED RWY 36R. NAV WAS SELECTED AT 500 FT DURING CLB. I WAS THE CAPT; PF. IAS 230 WAS SELECTED SHORTLY AFTER FLAP RETRACTION. AIRSPD CONTINUED TO ACCELERATE THROUGH 230. DUE TO THE INCREASING AIRSPD; I SIMULTANEOUSLY ATTEMPTED TO COMPENSATE FOR A WIDER THAN NORMAL TURN AT GVINE. I WAS HAND FLYING. I ACTUALLY OVERCOMPENSATED AND TURNED .67 MILES INSIDE THE RNAV TRACK. THE AIRSPD INCREASED TO 250 KTS PRIOR TO THE AUTOTHROTTLES BEING DISCONNECTED. THE FMS DISPLAY CONFIRMED IAS 230. RATHER THAN TROUBLESHOOTING ANY MORE; THE AUTOTHROTTLE WAS DISCONNECTED; THEN AFTER A MINUTE OR TWO; RECONNECTED. THERE WAS NO TFC CONFLICT AS A RESULT OF THIS DEV. I HAVE SUCCESSFULLY FLOWN THESE RNAV DEPS BOTH BY HAND AND ALSO WITH THE AUTOPLT ENGAGED. HOWEVER; IN THE FUTURE; SHOULD AN ANOMALY OCCUR SUCH AS THIS ONE; I BELIEVE THAT HAVING THE AUTOPLT ENGAGED WILL GREATLY REDUCE THE WORKLOAD; AND MINIMIZE THE POSSIBILITY OF DEV; SUCH AS THIS; FROM OCCURRING.