Matt, Bruce - thank you! I believe I found the problem - Artemis does seem to choke even on 200 paths, especially with the "fit background" option on. At least it does on my office PC - relatively old and weak machine, will start begging for a new one shortly. Sometimes it would generate a long error message starting with "use of uninitialized value in abs at artemis line 12984."; sometimes there is no error message but results would look really strange. Had me so confused that I started to doubt my fitting method. I have since trimmed down the number of paths and variables to a minimum (didn't need that many anyway), and the problem seems to be solved. Thanks again for your responses, gentlemen! Vadim.