If I swap chip with 12F1840, will problem still exist?
more than likely , you need to understand and remedy the situation.

http://www.picbasic.co.uk/forum/showthread.php?t=15843&highlight=shadow
http://www.picbasic.co.uk/forum/showthread.php?t=15013&highlight=shadow

and post your code and a schematic so we don't just keep guessing