[gradsusr] Strange situation about "Logic error 8 in gxshad"

[Cypresslin]

Hello, GrADS users.
I have encountered the following error messages while running my script.
 "Logic error 8 in gxshad"
 "Error in gxshad"

Googled this error message,
found out that previous posts point out this error might be caused by
invalid numbers.

My script contains a "do while" loop (T=1~6), it prints out 2 pictures
in one loop.
When running this script, the error message shows up after the first loop .
The first loop seems okay. So I removed the do while loop.

Okay, the strange thing is:
Start up GrADS -> Run Script -> (OK) -> Run Script AGAIN without any
modification -> (Not OK)
But:
Start up GrADS -> Run Script -> (OK)  -> quit -> Start up GrADS  ->
Run Script -> (OK)

What's wrong with it?
Thanks!
(I'm using GrADS Version 2.0.a9)