I have had a similar problem with my 1600.  I run ACP...everything runs along fine...would shoot a series of R, G, B frames.  All of them are fine except there will be several in a row where the mount oscillated in RA-the star is there but there are "ears" where it oscillated.  Intermittent, which as you know are the hardest problems to DX.  No clouds or anything.  Have you seen what you mount can do unguided as a base?  Is everything smooth meaning cables and such?  Imaging train snug?

Mike J. Shade

Mike J. Shade Photography:

mshadephotography.com

In War: Resolution

In Defeat: Defiance

In Victory: Magnanimity

In Peace: Goodwill

Sir Winston Churchill

Already, in the gathering dusk, a few of the stars are turning on their lights.

Vega, the brightest one, is now dropping towards the west.  Can it be half

a year since I watched her April rising in the east?  Low in the southwest

Antares blinks a sad farwell to fall...

Leslie Peltier, Starlight Nights

International Dark Sky Association: www.darksky.org

I need some advice I am getting bad images 80% of the time due to "bouncing" stars. It's a zig -zag pattern. It seems to occur on the East side of my AP 1600 mount. I have attached an image. Has anyone seen this pattern and any suggestions?