Your problem is several fold. But ol Skeet has been working on Pop Rs since 1986. We can get ya fixed up. First, are you a Pop R shaver? Shaving the lip is fine. It really only affects the popping action of the lure. If you sand the sides of the lure, then you are giving yourself alot of unnessary problems. Sanding the side of the lure thins the profile of the lure as it sits in the water and REALLY decreases the bouyancy of the lure. You are actually removing bulk that the lure needs to float. It also will kill the walk the dog action that you can get out of the lure. Since the sides are flatter and shoving more water, then the lure will not move from side to side as much as it is capable of. If done properly the face of the lure will give you all of the splash that is needed for this bait.
Devcon probably did cause you the problem that you are experiencing. If you put a thick coat of it on the lure then you did add alot of weight to the bait. Use clear fingernail polish. It really is very tough and will hold up to abuse. It also works well on plastic cranks, thread on dressed trebble hooks, fishing rod wrappings, etc.... The next thing was the water temperature. Cold water is denser than hot, therefore, the lure sank. I will bet that the lure that sank will float come april or may. I will bet that there are alot of bait makers out there that have baits that will slowly sink sitting in there box right now. It is just that most folks don't fish when it is 16 deg. outside. Most of them will ever know it. I have taken all of my lures to extremely cold water to test them to know what it takes to make a lure for a customer that is either an all season crankbait or a seasonal crankbait. But for now you really do need a thinner, lighter, clearcoat.
The sagging is caused because you are not spinning the lure long enough. I have posts on the site covering both temperature and spinning times of Devcon coated lures.
Skeeter