Have you verify the tension?
Is it the same with another design?
Sometimes letters are difficult to embroider.

Perhaps you need a heavier stablizer.

Sounds silly but rethread you machine both top and bottom, making sure you get the thread tight in the guides.

is the design to small & or to dense,I agree with eveyones suggestions also the bottom weight of thread must match the top! hope you have better luck, Hugs Loralye

If you've re-threaded the machine on top and at the bobbin and also if you changed your needle make sure it's not in backwards. Sounds silly but it can happen. If that doesn't solve the problem it could be out of time. Hope one of these ideas help if not you may have to take it to your service man.

You need to lower the density of your letters. You are stitching one stitch on another....In lettering, the density should be lower than for regular designs.

Try to make the embroidery less dense by changing the type of stitch which you are embroidering in to a larger stitch length or a different stitch type(i.e. from satin to step stich) I hope this helps somewhat.***

Are you using bobbin thread?

Thanks everyone for your answers. I will go try some more.