When I print a somewhat small part it goes fairly well but whenever I print a big part I always end up with cracking. I changed the temperature around from 210 - 240*C and I'm using a .35mm layer height with a .5mm nozzle using slic3r. I also get layer seperation when using skeinforge as well. I'm using plastic from Protoparadigm and tried different colors but they all do it.

Are you sure the flow rate is steady? Sometimes the odd missing filament can cause cracks. For example if the extruder acceleration is too high or the hobbed bolt is slipping occasionally.

What width over height are you using? Bigger sticks better, so does higher temperatures, lower speeds and more outlines.

Putting the printer in a box to raise the air temperature helps a lot.

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From the temperatures you mentioned, I assume that this is ABS? Are you using a cooling fan? I find a fan to be very important for PLA, but with ABS it causes the upper layers to shrink too quickly and causes cracking. I don't use a cooling fan at all when I'm printing ABS.

I'm letting slic3r pick the width over thickness and I'm using ABS with a heated bed and no fan. I'll try increasing the width over height and go from there.

I'd actually try hotter than 240, and see if it clear up.
Have you tried a different roll of filament?
I have a roll of Green PLA that does the same thing until you get the temperature a good 20 degrees higher that what looks like good extrusion.

I tried a few different rolls but they all do the same thing. I'll try it even hotter but for the time being I switched to PLA to finish the part I'm trying to make. You can actually hear it crack as the layers cool and the layers look pretty good up until about the stresses are too much for the part. Small parts never have a problem, only larger ones.

Do you have a window open or airflow from a nearby A/C or something? Try making a simple "heated" chamber and protect your build area. Cardboard or even paper works fine to test it. You could even just put down a big cardboard box over the entire printer, just take care it doesn't get too hot.

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-Nudel
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