Miles? Do you have any loud noises under the hood? I've seen a car do exactly what you have listed and it ended up being a cracked exaust mainifold.
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1999 Forest Green GTP, 3.4 pulley, Magnusen snout, Thrasher CAI, Mod PCM, Eibach Springs, Koni struts, Pods Tranny Temp/Boost, Optima Redtop Battery, Tranny Cooler, Borla Cat Back, Ubend removed, Blazertech 3200's,