99 Merc Tracer P1506, P0455

Hey All. 1999 Mercury Tracer, 2.0 SOHC, Automatic, 124K. Purchased 3 mths ago after a lube shop replaced the head gasket. Check engine lights been on since the 2nd day. P1506 (IAC overspeed) P0455 (Gross leak) only codes left to fix. Idles btwn 1400-2000 warm or cold. When a/c compressor engaged, idle falls to 800-1000 but vehicle shakes very bad and (with brake applied) squealing from belt. Not enough vacuum at EGR valve to move the needle on hand gauge(at any time). Work completed= New plugs & wires-Almost all rubber vacuum lines replaced-ALL vacuum lines(rubber/plastic) checked and no leaks-Cleaned MAF sensor-Replaced Catalytic converter, PCV valve & both O2's-Removed & cleaned IAC-Throttle body-EGR valve & new gaskets-NO intake manifold leak found. IAC valve activates when unbolted & ignition turned to on but no change in rpm's when unplugged (installed). I hope you can help ! THANKS !