Well the Xtal of such high frequencies do not need burden capacitors. But even if connected should make no difference. The problem being repetitive could be thought of. One of the factors is certainly the PCB that you are using. Stray cap., track shorting etc. The best way is to split the issue. Mount the processor on a separate board with no other comp. mounted. See if the crystal response is still the same. I think it will not be.