Appending text to the end of a file requires you to open the file, i'm currently investigating if that actually involves reading the entire file.
The filehandeling involved with the RFLIN command do read the entire file every time the command is isued. I read all the lines of the file into a string array to be able to get a specific line from the file.
The large amount of text in the two textboxes might cause problems. I hope my test's will tell my if that is what is going on.
I will change things in the upcoming version so the textboxes only hold the last 100 or so lines to eliminate this as a potential problem.