good post Three3s ... + rep
on a couple of points you've noted ...
definitely if you can get number 1 in google for a product name you can get sales from it ... it's easy to do for a couple of days, but then the so called "google dance" hits you ...
and as for changing pages ... well, i've learnt that one the hard way too ... but your slip may not necessarily be due to your changes, it might just be the google dance ... if you're using blogger you may not be able to, but what i do when that happens is look at my copy of the old page and figure out what key elements changed - e.g. yesterday i discovered i'd forgotten the H1 (header tags) ... makes a diff, perhaps not so easy if you're using blogger ...
finally :
* if you haven't already and you're able to, setup google analytics to show you where your traffic comes from ...
* also use google's webmaster tools ... combined these really help to show you how google sees your page and figure out how to maximise traffic

