Re: Sudden High Surging Idle ?
Suggest to check the system for codes, running a KOEO/KOER self-test. Also check the TPS @op temp, for flat spots/spikes on the output voltage, Ign On, while slowly moving the accelerator blade from closed throttle to WOT, back and forth, this condition could also cause erratic high/low idle problems. The other possibility could be a sticking IAC valve. My 2¢.
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ACRONYMS:
LUK=Let Us Know, GL=Good Luck, LMK=Let Me Know, JIC=Just In Case, BTSTDTRT=Been There Seen That Done That Repaired That and the classic... DILLIGAS=Do I Look Like I Give A S***
1986 Mustang GT w/T-Tops 
NOW: 5.0L Stock SB, X303 Cam, 700 cfm DP Holley, 289 '66 Heads, 1.7 RR, Richmond Gear 4+1 Tranny, Centerforce DF, 2.73 Gears
WIP: 3.90 Gears, LS Overhaul, Adj. LCA's & UCA's (Granatelli), T-Top weather striping and rear braces bodywork 
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