The following code works in browser and v0.6.19 as expected,

(function loop(timer){
    if(!timer) timer = 900;
    console.log('Timer is '+ timer);
    setTimeout(function(){
        var timer = Math.floor(Math.random() * (4000 + 1)) + 1000;
        //doSomething();
        loop(timer );
    }, timer);
})();

Will display,
Timer is 900
Timer is 1895
Timer is 2839
Timer is 4752
Timer is 4556
Timer is 2115

Under v0.7.10-pre(the only other version I tried), will display
Timer is 900
Timer is 4975
and hang there. Setting the initial value to anything above ~3 seconds will cause it to hang from the start