You should float eye level with a normal breath AND ALL the air out of your BC. When you exhale you should start to slowly sink, no drop like a rock. Buoyancy control is a slow and deliberate process. Proper weighting is important but your understanding of the physics of buoyancy are much more important than being dead on with your weights. If you understand buoyancy you can dive with any BC or amount of weight, with in reason of course.
There are 2 basic problems I would suggest you consider.
Many divers tend to unconsciously fin when they are trying to descend feet first, they are finning up while trying to descend. To compensate for the finning divers will add more weight and dump the air in his BC, once the effects of the finning is overcome the diver starts to drop like a rock. To stop the decent a lot of air is added to the BC, in almost every case way too much air. About this time is when problem 2 comes into play. The simple fix is to stop finning and be properly weighted.
Problem 2 is more difficult to deal with. One of the most difficult concepts of buoyancy to learn is it's delayed response to changes. This is most likely one of the major problems you are experiencing. We are all use to things happening almost instantly, you press on the car's brakes and it immediately starts to slow down. This is not the case with buoyancy. When you add or remove air form your BC or take/let out a deep breath, it takes several seconds for the full effect of what you have done to take effect. New divers are way to impatient, they add air, nothing happens immediately so more air is added and maybe even a 3rd time. To make matters worse, each addition is bigger than before because "nothing is happening". Then the effects start to take hold and the diver starts to ascend. The accent starts to speed up, so the diver dumps some air, nothing happens, more air is dumped. About this time the delayed effects of dumping air starts to kick in and the diver is heading to the bottom fast. The result is yo-yoing up and down and having to constantly add and dump air. The solution is to be properly weighted and when you make a buoyancy adjustment, SLOW DOWN AND WAIT A FEW SECONDS to allow time for the effects of your change to take place. The art of buoyancy control is to know how much air to add/dump and when to do it to get the result you want.