Not sure how to fix it but I had 2 Mosin's that had the same thing.  I just got used to it.  I've read that some people have swapped in a Finish M-39 trigger which gave more of a 2 stage feel.  However I suggest you just get used to it.  

There is a single piece of metal that is both the trigger spring and the bolt stop.  I replaced one on an M44, and it fixed much of the play in the trigger.  I paid $15 (ouch! for a $35 rifle) for one at a gun show.  

Tapco doesn't have them, but I have seen them for sale at:

[url]http://www.buymilitaria.com/mn_htm_files/mosin_nagant_rifle_trigger_spring_&_bolt_stop.htm[/url]

for $12.  There's a picture of the part so you can see the one I'm talking about.

look around on these sites...

[url]http://www.gunboards.com/[/url]
[url]http://www.gunboards.com/forums/UltraBoard.cgi?action=Headlines&BID=28&SID=138803[/url]
[url]http://www.mosin-nagant.net/[/url]
-bricklayer