Many table tops have been ruined with a belt sander.
I'm just saying.....
Super true statement above! Suggest after wood is FAIRLY even at the glue joints, you put the belt sander down and walk away quickly. Pick up a random orbital sander like Steve suggests. A slower stock remover, but with infinitely more control. If you are going to woodwork, and especially if you will build slab type projects (tables, cabinets, boxes, etc...), an ROS will be invaluable. I have six or seven of the little devils, each with a different paper for easy switching.
Try 120 grit in the ROS first and see if your problem areas begin to smooth out. Once the entire slab has had a good going over, sight down the length of the slab. If you see dents, divots or uneven spots (usually from not keeping the belt sander moving), you might be forced to go to 80 on the ROS. Again, leave the belt sander in the cabinet. When the project is smoothed out sufficiently, trade back to 120 again for a bit, then move down to 220 for a baby's butt finish.
Finish preparation is one of the hardest things in woodworking (at least for me), but is probably the most important. We all want it to go fast and be done, and some of us will go to great lengths (and expense) to speed the process. But a FINE woodworker will take great care in his prep work. Time means nothing. Details matter. And it shows off in his finished product every time.