If I run 2 this suckers:
cat /dev/zero > /dev/null &
and antlr under nice (nice java -cp ... org.antlr.Tool My.g) priority just for sure, it just gives out totally different code, shorter for about 15000 lines (diff). There must be some uglymoronic race.
EDIT: Ok, according to Terrence Parr (the antlr leader), it's not a bug from his point of view, see his comments below.
The trick is -Xconversiontimeout 600 on my machine.