If you are having issues with the top thread showing on the bottom, then it's probably an issue with the top thread tension. I'm inclined to agree with Dunster on the tension assembly springs, however, if you had it looked over by a sewing machine repair person, this should have been one of the things they checked.
I will say that if your new project has different top or backing fabric, or a different batt or you are using different thread from the last project you did, you will need to check and possible tweak the tension. Once you get it set correctly, it should be good for the whole project.
This thread outlines how to set the tension on a different brand machine, but you might try going thru the steps to see if it helps:
http://www.quiltingboard.com/main-f1...t-t114933.html