I had similar problem as you have mentioned. The important point in ensuring proper SU-8 adhesion to Si wafer is that the silicon wafer surface has to be extremely dry (highly hydrophobic). Generally I found it is useful to bake the virgin wafer at 120C for about 30 minutes to ensure any moisture on the surface evaporates. Then allow the wafer to cool to room temperature and then carry out the spin coating. I did the same and this worked. The baking you are doing after coating seems ok. I too follow a similar procedure. You do not need any adhesion promoter....just ensure the surface is completely dehydrated. Good luck !