Arduino frozen after 4 String concatenation

Projects Programming
3

This is still happening on Arduino Mega ADK with Arduino 1.0.3 for this code:
...
Serial.println("step 6");
logMessage= String();
logMessage=String(" Left: ")+String(straight_left1)+String(" : ")+String(straight_left2)+String (" = ")+String(median_left)+String (" L_Spd: ")+String(left_speed)+ String (" R_Spd: ")+String(right_speed) ;
Serial.println("step 7");
...
After few iterations, everything stops between "step 6" and "step 7"

Did anybody found a solution for this?