Sounds like a booster :|
Really, having a small kid in a booster really does tend to be MORE annoying than a harness.
It sounds like you know exactly what';s going on, and sadly, I dont know of a way to *fix* it except to do what you are doing.
It's very possible that a booster with a different design/belt guide angle would not be catching the belt in that way, but if you have a frontier, you probably arent looking to buy another seat. A different...