I also experience that problem. I also dont know how did I manage to solve mine. But I think it is beacuse of the number of characters you sent. I also used a <SoftwareSerial.h>. But in your code, you send all the data thru serial and what I did is i just send data to serial what is just necessary. I wanted to put the code here but its bit long so if you want to know how I did mine here is the link. I hope it helps and ignore if it dont. Thanks.
http://thinkingparts.blogspot.com/2015/08/home-automation-diy-version-internet_23.html