Cloning with LambdaGEM-12

[Chutney]

LambdaGEM-12 Xho I half-site arms cloning system.

In this system LambdaGEM-12 has been digested with Xho I, partially
filled-in with dTTP and dCTP and dephosphorylated.  This allows it to
ligate specifically with Mbo I or Sau 3A I digested genomic DNA which has
been partially filled-in with dATP and dGTP.
   Using this cloning system (from Promega) I have been obtaining
unusually high levels of background when vector DNA is ligated and
packaged in the absence of insert genomic DNA.  Typically 20,000
pfu/microgram of vector DNA.
   Has anybody else had similar problems??  I would appreciate any advice
available.