High Mol. Weight Peak from Library on Agilent Bioanalyzer

[MWN]

I made some TruSeq DNA library using Illumina adapter from sonicated FFPE DNA and ran the amplified library after AMPure purification. I have seen the extra high molecular weight peak > 1000 bp. The position of peak looks different from the extra peak described in some earlier posts, which is double and triple in the size of expected peak.
I did not do a size selection. I am going to do hybridization enrichment using the libraries. Just wander about the impact on on-target rate.
Do you have any explanation on the extra peak? Uneven sonication for FFPE DNA even I observed a main 150 bp peak? A running artifact of Bioanalyzer? Concatemer?

[MWN]

I like the idea of ssDNA heterduplex as discussed in

http://seqanswers.com/forums/showthread.php?t=559

Perhaps increasing primer concentrations and/or reducing PCR cycles will help.