I have a 2006 7520 with autotrac running on sf1 and a 3000 receiver.  A year or more ago, after updating to the new frequency, the tractor would act goofy (wag the front wheels back and forth several times) when you engaged if you weren't fairly close to the line and heading (which it never did before).  It started the day we updated the frequency but it could be coincidence.  Later that season doing tillage work it wouldn't hold a straight line.  It would do a very subtle weave.  You could sense it just enough to make you a little sea sick but the evidence showed clearly in the disking pattern.  The dealer did exhaustive tests trying to figure it out but they didn't swap any of my components as they should have.  Deere wanted to say the steering valve was bad...the valve probably had 1200-1500 hrs on it.  They basically gave up and the salesman was sure that his hunch that something was wrong with my itc receiver was correct.  So last winter I caved in and  bought a new 3000.  It truly is better than the itc like he said.  Tractor steering problem went away.  Fast forward to this week...now the tractor doesn't seem to try very hard to hold the line.  I haven't changed any settings.  I'm pulling a 12' Perfecta II (3pt field cultivator) ahead of a 12' grain drill on level ground that has been plowed and worked once or twice with the perfecta before the planting trip.  I run markers on the drill planting back and forth on an A B pattern.  I am going between 6 and 10 mph.  It does better the slower I go.  Sometimes it stays on line pretty good and sometimes it lets itself drift off what seems like too much (6" on the screen, more than that looking at the marker line) without really trying very hard to get itself on line.  This is more of a drifting offline to one side or the other without correcting soon enough rather than the weaving it was doing before.  Generally autotrac can keep up better than a person can manually but in this case if I take over I can do a much better job staying on line manually...and my arms don't work very well.  I realize that there may be some draft forces from the equipment I'm pulling which would make the autotrac work hard but if I can do it manually surely autotrac should be able to keep up. This is happening in an open field not near trees with a full yellow bar.  So, what do you think is going on now??