Sue, are you using thread made for hand sewing, by chance?
If you're using thread made for machine sewing, try changing to a new sharp (as opposed to a ball point or a universal) needle and see if that improves the sewing.
If that doesn't do anything, change to a new sharp needle one size larger and see if that helps.
A 50wt thread made for machine sewing should do just fine in the situation you describe.