I would first suspect some sort of powersaving feature. Try continuously using the phones interface locally (reloading webpages or something of the sort), and I bet you'll notice that your FTP server is always available from other computers.
I say this because I've noticed what seems to be similar behaviour with servers running on my hacked kindle3 (wifi only). They stop responding until you generate traffic from the kindle, which kicks the wifi card up from some sort of lower power level, or something.
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I would first suspect some sort of powersaving feature. Try continuously using the phones interface locally (reloading webpages or something of the sort), and I bet you'll notice that your FTP server is always available from other computers.
I say this because I've noticed what seems to be similar behaviour with servers running on my hacked kindle3 (wifi only). They stop responding until you generate traffic from the kindle, which kicks the wifi card up from some sort of lower power level, or something.