Timing slipping after timing belt replacement

After replacing the timing belt, the timing slipped.  I assumed that this was a problem with the crankshaft sprocket slipping so I realigned the crankshaft pulley and replaced the crankshaft pulley bolt twice.  The first two times I followed the recommended tightening procedures (torque to 100nm, wait, tighten 90deg, wait, tighten 15deg, recheck timing), the last time I used an impact wrench for the first tightening. Every time it start well than starts to slip timing after a few miles.  Any ideas would be great.