1998 bobcat skid steer 763 has a surge problem

go to the right side of machine and take cover off that is where the timing belt is and see if it is broke or something is holding it up

straight stitch is the correct setting

Other than having your machine serviced, you might try putting a piece of scrap fabric (butted up to the end of the fabric you are sewing) and continue sewing onto the scrap fabric. (Sewists do this frequently at the beginning of their fabric and it is usually called a leader. It helps eliminate thread nests on your project when beginning to sew.) Hopefully, then when your fabric swerves, it will do it on the scrap.

The "dial" in the back is for decorative stitches which require a "programmer" to be inserted with the stitch formation you want to form. For a straight stitch no cam is required in the back of the machine. The three dials on the front:

Upper Left: Stitch selector
Upper Right: Stitch lenght
Lower: Stitch width

Using the upper left dial select the straight stitch picture
Using the upper right, choose your stitch lenght
Using the lower, choose your needle position, left/center/right

Cotton reel on top of the machine, make sure presser foot is up, make sure takeup lever is at the top, take the thread to the left ot the machine going through the guides at the top, bring the thread towards you and straight down and round the big cotton guide and straight back up, right to left through the takeup lever then bring the thread straight down through any guides and then thread the needle

From the spool pin take the thread to the left and over the top cotton guide, bring the thread towards you and straight down in between the tension discs, make sure the presser foot is up so the discs are open, straight down towards the needle and round the first big cotton guide, straight back up to the take up lever making sure the take up lever is at the top, right to left through the eye of the take up lever, straight down to the needle going through any guides on the way, thread the needle front to back

Some machines are not designed to sew the straight stitch with the needle in the center position. Some are left or right position to do that and it is simply the way they were designed. Linda at Relics

As to your question about leather, the 6460 has a special stitch for sewing leather. Select Colormatic cam C and install in the cam hole at the back of the machine. To remove/add a cam turn the upper left hand dial on the front of the machine to the small dot. This disengages the cam. Gently remove the cam by pulling straight out from the back of the machine. Install cam C straight in, ever so slightly twisting it back and forth into the hole to find the sweet spot. You'll hear a click when it's all the way in.

To use Viking's leather stitch, turn the upper left dial to the color yellow. (Note: The slash line on the upper left dial is a stop. You can't turn past that point. Go in the other direction to get to the icon or color you want.) Turn the upper right and bottom right dial to yellow also. The upper left dial is what selects the particular stitch on the cam, the leather stitch in this case. The upper right dial selects stitch length, and the lower right dial selects stitch width. You can move the right dials within the color range. Experiment on a scrap of cloth to get the stitch size the way you want it. The leather stitch looks like a straight stitch with a block style edging to the right of the straight stitch. This is supposed to give a stronger stitch to the leather rather than a straight stitch by itself.
If you don't have the manual for your machine: Select which of the eight cams you want to use. All the cams are removed/installed by turning the upper left dial on machine front to the small dot. At the rear of the machine, pull/insert cam straight in or out and twist slightly as you push in or remove to find the sweet spot. Move the dial away from the small dot to lock the cam in.
Turn upper left dial to stitch type you want and match the other dials to that same color. Upper right controls stitch length, lower right controls width. Bobbin tension is controlled by the dial at front left of the machine, above the needle.
Cams A-C are the workhorse cams and the rest are embroidery stitches.Straight stitch, button and stitched zig-zag will work on any cam. These stitches do not rely on matching a color code. To use those three: To select straight stitch, on the upper left dial, choose the icon that looks like a buttonhole with a zig that trails off to the right. Select stitch length on upper right dial and turn the lower right dial to zero, going by number rather than color.
To select stitched zig zag, turn upper left dial to the broken zig zag icon (to the right of the straight stitch icon). Select stitch length and width.
To move the needle to the side for buttonhole stitching or other times when you want the needle off center, turn the upper left dial to the button icon. If stitching a button hole, set stitch length to zero and lower right dial to desired stitch width. Push in the small white button that's beneath the lower right dial in order to prevent the feed dogs from moving the fabric forward. Pushing that button in is also handy for putting cloth under the foot before sewing as it prevents snagging and is used for tacking.

Is your bobbin winder button pushed all the way to the left? Sometimes, it will stick and not let your machine run. Move it back and forth a few times, then select a straight stitch and long length. Good luck

Put the bobbin on the short metal stick on top of the machine. Unthread the machine. Put the spool of thread on the tall, left side stick on top of the machine. Take the thread and wrap it counter clockwise around the circular, short, metal thing on the left side of the top of the machine, only 180 degrees. Thread the thread through the hole in the bobbin (which should be on top). Press the bobbin to the right, into the little white thing. Pull the turning wheel on the right side of the machine out. While holding the end of the thread, softly press the presser foot so that the thread goes around the bobbin a few times. After that, cut the end down to the bobbin. Now, press the presser foot down and wind the bobbin until it stops on it's own.