I create an application with three threads(access oracle database by bde tquery)

main program:random select,insert data;cycle:10S
thread1:random select,insert,delete data;cycle:10s
thread2:random select;cycle:60s
thread3:random insert;cycle:20s

five errors occur between 24 hours,longest interval:12hours;shortest interval:0.5hour,
error as following:
"Lost communication with SQL server.ORA-03113:..."
"General SQL error. ORA-03114:..."

ping database is ok during error;
the dba not find error like that in database log;

Client OS:WINDOWS XP
DevelopToolELPHI 6.0
BDE Ver:5.01
Oracle client:ver 8.1.7
Oracle DB:9i

why..?

can anyone help me?,thanks!