Pavel,

It would help if you would describe, what tools exactly do you use and under which circumstances does the described error happen:

• what sort of monitor are you using (preferrabli with a link)
  
• what is your '51 derivative and the surrounding hardware
  
• the shortest piece of program which produces the described behaviour (you might need to cut down pieces of your - possibly bigger - program to get something short enough to post here)
  

Otherwise, as we don't know what are you doing, we are unable to guess what's the problem - and you gave so little information that it is impossible even to give clues.

Jan Waclawek