I'd set the curb idle at 650-700, and set the fast idle to 850-900 (spec is 875 IIRC.) It sounds like your fast idle either isn't working or wasn't set correctly, but the procedure takes maybe 2 minutes tops and should be easy to find using the search function. All you'll need is a 1/4" wrench and a jumper wire.
1991 F350 Lariat XLT, 7.3l, 5-speed, crew cab, SRW, 3.55, 2WD. 115K original miles.
1990 Jeep Wrangler Sahara, 4.2l, 5-speed, Dana 30/35 w/3.07s
(for now!), 33x10.5x15 BFG Mud-Terrain KM2s on 4" Rough Country springs. Engine being rebuilt after an awesome afternoon in the mud LOL