Chip Reset not working on Arduino Mega2560 board

Recently I bought an Arduino board from internet (I assume it is a R1 or R2 because there is a U8 USB chip available)
After uploading my first sketch everything was working properly, but when the sketch became bigger then about 24Kbyte, I received a verification error at address 0x5700. When the same sketch was uploaded into a Uno board is was working like a charm.

My first impression was that the bootloader was not up to date, so i tried to upload a new one via the ICSP connection.
But I also receive a verification error on the second byte…
The Lock bits shown the following values “0xC1” and the Fuses “0xFF, 0xD8, 0xFD”.
(0xC1 is strange because 0x01 is a invalid combination)

I have a AVRISP MKII programmer and can get a JTAG and STK500 board from a friend of mine.

When I perform a chip reset, no error received, all lock and fuses keep unchanged,
Who knows how I can reset my uC into the factory defaults…

Thanks in advance,