[racket] Strange editor behaviour

I'm having a problem editing programs using DrRacket. This happened once 
before but went away after re-installing. [1].

I'm having the same issue again. Basically the behaviour of text 
selection is changes while I open and edit different files running a 
single instance of DrRacket.

If I select some text and then press SHIFT+Left instead of altering the 
selection based on the current cursor position, the selection is 
extended left from the beginning of the text currently selected.

Similarly for SHIFT+Right - it extends the text selection one character 
to the right from the end of the selection rather than the current 
cursor position. Same for SHIFT+Down, SHIFT+Up.

Is there some key binding or setting in DrRacket that controls the way 
text selection is handled that I am accidentally hitting?

It doesn't happen in any other editor - it's specific to DrRacket.

If I quit DrRacket and re-open it normal operation continues for a while 
again. I don't know if it's a bug or some setting in DrRacket that I am 
unaware of.

My current specs are:
Arch Linux 64 bit:
Linux 3.10.10-1-ARCH #1 SMP PREEMPT x86_64 GNU/Linux

DrRacket 5.3.6.
(This is a version I compiled myself - but it's the same using the Arch 
binary package - racket 5.3.6-1)

Can someone offer any advice?

Bhrgunatha

[1] 
http://article.gmane.org/gmane.comp.lang.racket.user/17034/match=bhrgunatha