A quick look at your code and possibly a bit of re-structuring will speed it up.
Your currently continually opening, writing and closing the SD file in the loop where a better method might be to wait until sensor reading is >100, note the time and sample x number of samples at y interval into RAM. You only need to log the sensor value (int) as your reading at a fixed time interval. A hundred reading will consume 200 bytes of RAM. After the readings have happened then open, write the values & close the SD card and print to Serial if needed.
Getting rid of the String class will probably help speed/stability issues as it is known to be buggy.