File System is too slow and makes Watchdog Reset

Hello

I have a problem with the file system.

If I write files on a SD card with the LPC2378.

If to many Files are on SD Card the File system is busy
until the watchdog makes a reset.

then I write a test rotine. The Result is
file 1-50 in 1 sec
files 51-75 1 sec 76-85 in 1 sec -100 files 3 sec -120 10sec

then 190 + 1 file 15sec
then +1 file 30sec
then +1 file 60sec

it seems the Filesystem needs exponential time to create a file with open file...

any one an idea or the same result.

//Test write file
volatile static int file_number = 0;

__task void t_save (void)
{ char filename[40]; char str[] = "Hello World";

int retv = 1;

file_number = 0;

while (retv != 0) { retv = finit (); }

for(;;) { os_dly_wait (10); // watchdog

sprintf(filename, "test%000005d.txt", file_number); file_number++;

FILE* fp = fopen( filename, "wb" ); if(fp != NULL) { fwrite( &str , sizeof(str) , 1 , fp ); fclose(fp); } }
}