We recognized that the output power of the pump laser was instable when externally triggered with 200kHz (stand-bye frequency of ESR). Therefore, we changed the coarse timing offset of the pockels cell trigger compared to the flash lamp pulse from 215µs to 219.9µs. 
Thereby, the output power increased from ~600mJ to 620mJ, and the PTP stability improved from 15% to 10%. The RMS stability improved from 2% to 1.2 %. With these settings we had UV pulse energies of 20mJ.