RPG program looping when run for PC

Ladies & Gentlemen / Boys & Girls -- 
A little back ground before I ask my questions.  We have an invoicing program 
that has been running since 1998. It was written in RPG III. About year ago we 
installed a PC running Client Access in the department that does the invoicing. 
 Now, at random times the program will go into a loop. We never had this 
problem when we were running it from a 'Green Screen" (twinax) terminal.  This 
problem has happened as offer as a couple of times a week or can go months with 
no problem. If they signoff (back to a signon screen) then back on and run the 
invoicing program, it runs fine.  It seems that the more the PC side is used, 
the more chances of the loop happening. 
Questions:
        1)      Does anyone have any ideals on what might be going on?
        2)      I have never used the IBM Debug on a program.  Can someone help 
with setting up an IBM Debug on this program when the loop happens again.

Thanks in advance,
William...