try printing two side by side.

basically the layers aren't cooling enough. The best solution is to add active cooling ducts to the print head. For pla it's pretty much essential. I also find it realy helps with abs as well.
Hell mine's always switched on (hmm, maybe that's the issue I have with taulman stuff - have to fit an on/off switch and find out)

The temperature you print at has a minor effect - what actually matters is how fast you can cool the hot plastic.
It's not quite the same thing.

But basically the bottom of a sphere is always going to be rough if you can't cool the layers properly. There's a thrad around somewhere where I've got comparison pics with and without the cooling duct.
It's a significant difference.