have you a cable adjuster down at the downtube cable stop? should also be one there to aid adjustment / tensioning.
also make sure the right colour of spring is in the correct brake arm and try and give them both the same amount of adjustment as harris says above, by turning the big flat nut (flat cone spanner works good for this, but not critical, and it doesn't take much to tension them anyway) whilst keeping the allen screw still, then tightening this down to keep everything snug.
since you've only a single cable and no gyro, etc, it really should be a bl**dy good brake you end up with, so it's probably something fairly minor giving you cr@p at the minute.
good luck!