My Janome 6600 will skip stitches (1 here ... then sew great for 6", then skip another - and so on) when there is a small bit of thread or lint somewhere in the bobbin area.
Take the bobbin and bobbin race out and give the whole thing a thorough cleaning.
Sometimes I'll leave the bobbin race out, thread my needle and rotate my fly-wheel to make a "stitch" (without bobbin thread). The act of the top thread moving through the machine and around the bobbin sometimes catches unseen bits of thread/lint etc.