I had a rebuilt motor put in it with a new WATER PUMP, RADIATOR, TIMING CHAIN, 2 O2 SENSORS, TPS, IAC, PLUGS and BATTERY. Starts fine, highways speeds fine, just bogs out when cold or warm at low speeds (0-15 mph) for about 5 seconds to 1 minute. If I let off of the accelerator the rpms are fluctuating between 300 and about 700 and will occasionally just die if I don't put it in neutral OR let off gas for a split second then go WOT. When I push the accelerator in normally the rpm's are at about 1K when they should be at 2K. But then it just picks back up again, and it doesn't do it again until I turn it off and restart it. Again, it still does it even if it's warm (I shut it off and get gas quick) and usually happens at about 0-15 mph but never at highway speeds. The mechanic that did all the work on it is at a loss right now, and has had it for about 5 months. By turning key on/off 3 times I get P0141 and P0161 codes (both downstream o2 sensor heater element malfunction.) Looks like they pertain to the O2 heaters, but not sure if that would cause this. There are 2 mini Cats. at the manifold both of which are about 700 degrees F. The last cat. w/ no o2 sensors is about 400 degrees F. Mechanic said fuel pressure tests fine at the rail.