There is no motion blur in the original scene because it a screen capture not something shot with a camera. It's gonna look funny no matter what you do in Premiere. You'd have to get into AE and know what your doing to get what your after.
Right-click on your time-altered clip and uncheck "Frame Blending". Instead of attempting to interpolate three frames into one (ie. 300% / 100% = 3), Premiere will skip two frames out of every three. Instead of the blurry look, you'll get a somewhat more stuttered look. Unfortunately, there's no free lunch--you'll have to deal with one side effect or the other, simply due to the math. Your other option is to find a program or plugin that does better motion compensation--you can probably achieve this for free with VirtualDub or AviSynth and the appropriate filter, or purchase a commercial plugin like Twixtor.
Thanks both of you.
Frame Blend unchecked seems to work