PCR anomaly

[Mark Edward Bowen]

My PCR reactions have suddenly become dominated by two well defined and
reproduceable small MW (~500bp) products.  It happens with different
buffers, enzymes, primers and templates.  The products always appear to
be the same size.  I can anneal at temperatures up to 72 C and only lose
my desired product.  I've tried hot start, touch down, Mg and
temperature titrations as well as varying template and primer
concentration.  I'm at a loss.  Any ideas?  I use a Stratagene
robocycler.
--
Mark Bowen