Compression numbers:
1) 173 psi 97.2%
2) 170 psi 95.5%
3) 174 psi 97.8%
4) 165 psi 92.7%
5) 178 pis 100%
6) 148 psi 83.1%
I'm going to venture six is related to four and guess head gasket?
edit: when looking at it percent wise that doesnt seem logical.