Offset (plugin) time expression question

Hi all,

I'm using the Offset plugin to cycle an image continually though the comp.  There is a point where I need the cycle to slow down between specific frames, but between the frames where it should be slowing down - it going in the opposite direction really fast.  After passing the second keyframe, the image then moves in the correct direction, at the new slower speed.

The comp is 1920x1080.  On the picture's layer, I've added Offset and a slider control I've renamed to 'Offset Controller'.  On frame 23, there's a keyframe on Offset Controller of -700; on frame 33 theres another keyframe of -100.

I've added an expression to "Shift Center To" on the Offset plugin, which is:

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x=960;

y=540;

[time*effect("Offset Controller")("Slider")+x,y]

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I can't figure out why between those keyframes the picture goes in the opposite direction really fast instead of simply slowing down.  Any help would be greatly appreciated.