If you have an ST60 'Multi' or a volt meter, check the voltage you're getting through the circuit whilst it's doing the steering. I had a similar problem (albeit with an ST4000+) and discovered an occasional drop in voltage from nominal 12.6v-12.8v down to <10v. This only lasts a fraction of a second but is enough to screw the pilot up. Was caused by (as Salty suggests) poor wiring so replaced with proper tinned cable and no problem since... good luck.