Really the difference between the 3790 original and modified is not as worrisome, but it could not exist. The question is "academic" and practice: understanding the exact cause and later finding a solution. It would be perfect to have the 3790 modified with the same level of precision of the original. I don't believe that the WAAS has direct link with mentioned problems. These models do not have WAAS / EGNOS available possibly because your level of accuracy is more than needed for civilian and road use (good hardware and sophisticated algorithms correction). I hope to have time in the next days to do some experiments, nothing like practical and methodical testing to confirm, whether or not, our views and hypotheses ..