Get into the habit of developing your paper for a full three minutes. A minute and a half isn't generally enough.
5-8 seconds is also too short an exposure time. Just a slight bit more or less could have a huge effect on your print; so short a time also doesn't give you any time for dodging/burning. Try stopping down your enlarging lens so that you achieve an exposure of at least 15-20 seconds. That way you can more easily fine-tune your exposure if you find your prints are too dark or too light.