Homebrew PCBs

I am using HCl (31%) + H2O2 (30%) + water in the ratio of 3:1:3 at 
room temperature with manual agitation (rocking) to etch some boards, 
but I'm having some problems. The boards seem to take a long time to 
etch (>15 minutes), and rather than etching evenly, the edges of the 
board etch first, then the perimeter of the non-etched area gradually 
decreases. I'm concerned that the etching from the edge will undercut 
the traces. What am I doing wrong? Other people report even etching 
in 1-5 minutes with this mixture, so I must be doing something wrong! 
Thanks for any help.

Mark