Fresh rebuild with just 250 miles is not even broken in yet, do a compression check to determine if the rings have seated.
Any leaks in the exhaust prior to the o2 sensor will cause the engine to run like crap, 16V's are notorious for cracks in the stock mani prior to the o2 sensor, this introduces fresh o2 into the airmass fooling the o2 sensor into thinking the engine is running ultra lean and dumps alot of fuel to try and correct it.