A not so common problem that could be causing this is the tensioner is not "tensioning" sufficiently, ie, not moving enough to take up the slack in the belt. Remove the belt again and loosen off the tensioner mounting bolts, using a suitable lever, push the tensioner to the end of it's travel, see if it's sticking or fouling. Re-align the cam and crank timing marks, refit the belt, and using the lever, push the tensioner as far as it will go to take up the slack, then tighten the tensioner bolts, turn the engine over TWICE, BY HAND, CLOCKWISE as you look at the front of the engine, to make sure everything is ok, then re-check cam and crank timing marks. If all is ok, the timing marks should line up and the belt should be fully tensioned. If you still have slack in the belt, it's quite possible you have the wrong replacement belt, but as you said, you are getting the same problem with the old belt, so, one of two things, the tensioner is not adjusting enough to take up the slack, or, you have routed the belt incorrectly. Re check the belt routing against the diagram in the manual. With regards to the teeth, on the belt, the grooves in the cam/crank pulley should tell you, if you have square grooves and round teeth, or vice versa, it aint gonna work. If I remember correctly, they should be round teeth on the belt.