I've just started using grabserial and I've not reproduced your problem. Current grabserial (1.6.0) does have an option to autostop your program.
-e, --endtime=<secs> End the program after the specified seconds have elapsed.
In case it could help, this is my command line (I've downloaded it from git)
./grabserial -v -d "/dev/ttyUSB1" -b 115200 -w 8 -p N -s 1 -t