I bought the 2g data travel pack. On arriving in Hokkaido, I have been able to connect on normal roaming but not data. I get connected, and then it disconnects, and keeps on having that instability.
As a result of this, I also ended up activating the 2g data pack once more as I thought it was my problem. I had no problem with this data pack when I went to Philippines before.
The network connection is very much dependent on our roaming partner's network while roaming. Quick check, did restarting your handset help at all?