programmer is not responding (the ubuntu 10.10 in virtualbox,the host is win7)

Development Tools IDE 1.x
1

Hi,
I build the arduino environment in ubuntu of the virtualbox.
When I try to upload the code .
Sometime is OK,but always cause error,like these:
avrdude: stk500_recv(): programmer is not responding.

Can anyone help me with this?
Thankyou.