D50 and AF
I had the same issue + a flash over-exposure issue with my Tamron 17-50 copy and returned it for a Sigma 17-70. Much better but in all honesty I wish I'd taken the 18-70 Nikon lens. You'll not get great AF with these lenses unfortunately and there is a huge difference to say my Nikon 50m in AF speed.
Make sure the contacts are clean (clean them with rubbing alcohol) If you're shooting in a very dark place I'm afraid you're out of luck as AF is just poor. One way I've gotten over it is to mount a SB-600 flash unit wit hit's own AF assist lamp. It does help but more often than not you end up bouncing the flash from the ceiling.
Sorry can't be more helpful.