You need to make a special MMS for the slim.
It's different than the normal one for the phat, check google for guides.
I've also heard of some issues with the PSP Slims video card when using pandora, try hitting x after the green light comes on and see what happens.
I've Hacked a few PSP slims and havn't come across this problem, but who knows, it might be your issue.
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