Poor stitches

Question

Bernina 125. Top thread stitches are good but bottom is good for awhile thenthe stitches look like they are sorta twisted and they are not as "snug" as they should be. Like it isn't sewing straight. is nothing Something is wrong as it did not do this before. Does not seem to matter how the tension on bottom or top is set. Have tried a new bobbin case, needle, cleanedit out again as well as having both shops clean it out. It has been threaded numerous time also. Two different shops have serviced it and it still doesn't sew properly. HELP????????