Stan,  The Simplex system is very good, but it does need to be adjusted
correctly.  First, the wood friction cup wheel and the wooden "T" bar
must be clean, with no oil or other residue to make them "sticky".

If the edge-tracking "ears" go in too far, the wire leads from them
need to be bent so they don't ride so far in, but it doesn't take much
to make a change.  They are made with a bending location just for this
adjustment.  John Tuttle's article show this.

The mechanism will "hunt" a little bit but it shouldn't be going back
and forth continually.  A sticky friction wheel can cause this, as can
the use of nylon monofilament fishing line.  I like using a multi-strand
fishing line -- it doesn't stretch and it is very flexible.

David Dewey