Chuck, you may have, but I didn't see you explicitly state whether you also tried a new battery. If not, maybe do the "spin" test on your battery. Place it on a smooth flat surface and try to spin it. If it easily spins from being slightly swollen then it may be just a bad battery issue.
Otherwise, look for any signs of corrosion on the board. Sometimes there will be corrosion even if you are not aware of any water damage event. I have come across a few cases on Samsung boards where corrosion on components that control USB charging effected the charging.
Finally it may well be that you are using the wrong charge port assembly. I believe that an N900T (T-Mobile) charge port will work on an N900A (AT&T) but that the reverse is not true - an N900A does not work on an N900T. I was thinking that the differences were just in cellular compatibility but there may well be some differences in USB detection and charging .