Choppy animation RAM preview & rendered

Question

Hi
I'm using After Effects CS5.5 Version 10.5.0.253
i'm working on an iMac 2.66 GHz Intel Core i5 -- 8 GB RAM, OXS Version 10.7.4.
i'm creating a fairly simple animation in After Effects, manipulating the position and scale of a still image. i've imported a JPEG i created in Photoshop. my problem is that i have to start zoomed in quite far on a detail of the image, and then zoom out all the way so i need to keep it pretty hi-res in order to have a decent quality when scaled at 150%….the smallest file size/resolution i've tried that has not gotten pixelated at full size is a JPEG 4667x3121 pixels @ 200 dpi resulting in a file size 1.4 MB
but the animation is still jumpy and choppy -- both in RAM preview as well as when rendered.
not sure if it makes a difference or not, but i have motion blur turned on
any help would be appreciated -- how can i make the smoothest animation possible?
is it exporting the still image in a different format or size or does it have to do with something i'm doing inside After Effects?
thank you

Answer 1

i've attached a number of screen shots, and i hope they help understand my project and problem better. sorry for the insufficient information so far, and thanks for offering to help.
here's a more detailed description:
the animation is 24 seconds long. starts zoomed in on a part of an image -- and as it pans around it in a circular motion, also zooms out.
attached photos called AE project position 1 - 3 show the beginning, middle and end points of the movements. you can see the keyframes for both position and scale. Rick, i took your advice regarding scale and used a file size that at 100% is almost the size i need. rather than zooming in to 150%, now i only have to start at 116%. in the first screenshot, i left the preview size at such a tiny zoom so that you could see the rather large size of the source image i'm using....much larger than the 1440x900 frame.
i have tried using keyframe interpolation and the curve on the position parameter to smooth out the speed. screenshot called AE project position curve shows this.
of course the problem is that if i do 'rove across time' with keyframe interpolation on the position keyframes, then i have to change the placement of the scale keyframes in order for the movement to still work as desired.
i have turned on motion blur on the layer.
i have then pre-composed the layer and in a new composition, added Timewarp effect (for the motion blur only). as you can (screenshot called AE project composition with effects) i set the shutter angle really high to get lots of blur to try to mask the choppiness.
i pre-composed the layer so that i could turn on the little rasterize button. to be honest, i don't know what this does or why it's not available on a layer before it's pre-composed, but it was recommended to me to have it turned on.
when i RAM preview that at about 25% display size, it is ALMOST perfectly smooth -- minus a couple small jitters that are practically negligible.
when i RAM preview at 100% it is very choppy.
i have tried rendering a few times with different settings. i have tried a few with Photo-JPEG codec at about 75% quality, both at full size as well as half (i'd like to keep it to 1440x900 export if possible)
i have also tried rendering to Apple ProRes 422, also 75% quality, tried both full size and half.
when i watch any of these rendered results in Quicktime 7, they are jittery. some more than others, but nothing has been satisfactorily smooth...
hope this helps understand my issue a bit better. if you have any advice about how i can make this the smoothest possible, i'd appreciate it.
thanks