I've got a few articles on pedal technique ... such as this one ...
http://www.drummercafe.com/education/5-minute-lessons/;action=display;threadid=3995Your foot may just be slipping because you are still feeling a bit awkward, and thus trying to hard ... causing the foot to not stay relaxed on the pedal ... pushing forward.
Be sure to spend a lot of time just with the pedal ... playing clave in your case. Put on some music and play-along ... but only the clave! This will allow you to focus on your technique while playing music, but not being so overwhelmed, like you would if you were also playing the congas.
I do this type of practice routine a lot ... especially if I find that I'm having difficult, not fully relaxed, or simply not grooving. Isolate the issue and get comfortable with the technique.
Although this may not fully solve your problem, it certainly is something to keep in mind ... and at least rule out as a potential issue.
