When it changes from straight stitch to sepintine and back to straight it skips lots of stitches. Just paid $300 for a new shaft.

try these suggestions & see if they help

Sewing machine skips stitches
1.needle inserted incorrectly--flat side of the needle faces the shaft
round needle--groove of needle faces away from the shaft
2.Correct needle insertion is mandatory for proper use.
Raise the needle bar to the highest point, then loosen the clamp screw to remove the needle. Place the new needle in the clamp with the flat side of the needle facing the back of the machine.
If round needle--the groove in the needle faces away from the shaft
Push needle all the way up to the stop or as far as it will go.
3.possible bent needle--lay needle on flat surface & roll needle over, if the needle does not roll good -replace
4.blunt needle point--needle makes a popping sound when needle penetrates the fabric--replace needle
5.needle not all the way up in needle holder--loosen needle clamp screw & push the needle up until it stops then tighten screw
6.incorrect threading--one step missed in sequence of threading can cause machine to skip stitches
7.wrong needle in machine--check instruction manual--some machines can use a generic needle with no problem--some can't--
8.size of needle & thread not compatible--
Using the wrong type needle can cause problems,,
A needle to fine for heavy fabric can bend or break when it hit's the fabric
And a too large needle can make puncture holes in the fabric and pull unevenly while stitching