FSG reports when being run in xml output option is ending in error

i have defined 6 FSG reports. Out of this two reports, when its run in xml output option is ending in error.The other four reports am able to run through Reports manager and take the output as excel or pdf etc. These two reports have No issue when its run in the text format and the data also holds true. the only difference that i see in these two reports and the other four reports is that i have used standard column set for these two reports and for others i have used custom column set.