不是 means “from time to time", "sometimes", or "occasionally"。I know that's a pretty broad meaning, but you'll get the idea from context. 不时 seems to be used to less like the english "from time to time" (though it can be used that way apparently) and more like the an action that that happens a few times, or over and over, sporadically over a period of time. Example:
My dictionary: very english like use " 她不时来给我找点麻烦” She comes to bother me from time to time. (Yes, I know why this english translation is troublesome.)
nciku use, for example, more chinese usage
他的讲话不时被热烈的掌声打断 which they have translated as "His speech was punctuated with warm applause." which is ok, but the 不时 here really means 'he was interrupted during the course of the speech a few times by "warm applause"' or "a few times during the speech he was interrupted by "warm applause"'. At least, that's how I read 不时。 I think there's a fair or at least small amount of freedom here and in context you get the general idea, which with chinese is sometimes as much as you can hope for.