Success! I changed my code to this and got a smooth sweep from 500 up...

Code:
byte msb = 0;
byte lsb = 0;

void setup() {
  Serial.begin(8192);
}

void loop() {
  delay(10);
  lsb++;
  if (lsb > 254) {
    msb++;
    lsb = 0;
  }

  byte b2[5] = {0x0A, 0x58, 0x00, msb, lsb};

  unsigned int x = 0, sum = 0;
  for (x = 0; x < sizeof(b2); x++) sum += b2[x];
  byte cs2 = ( 256 - ( sum % 256 ) );
  Serial.write(b2, 5);
  Serial.write(cs2);
}
Now to figure out what I was doing wrong to begin with. It's gotta be the LSB math?