HD external USB long loading time or freezing

Hello

My LACIE 250 GB external USB 2.0 drive takes long time to respond sometimes and sometimes the read/write lamp goes on and win XP doesn't respond for a while. I've had the error message "delayed writing failed" a couple of times but not anymore. My solution to freezing was to unplug the HD if the freezing time was long. I've tried another USB port and also verified that virtual memory and write caching are disabled. I've also performed a surface scan with no errors. Haven't found any specific diagnostic tools for the HD but did a Passmark seq read test with 10MB/s which sounds ok to me. The drive contains fragmented files mostly large ones (> 700 MB) and 6 GB is free (ca 2 GB when problem occured).

Please help