It’s amazing how one show can be milk-out-of-your-nose funny one week, then just dull and lifeless the next.

The same goes for a show that was pretty lame last week and quite interesting the next.

I’m talking about “Family Guy” and “American Dad,” the middle block of Fox’s “Four-Fathers of Comedy” (get it?!? Ha! Well, not really that funny after the third time it’s advertised as such on TV.)

Last night on “Family Guy,” Stewie and Brian host an afternoon radio talk show. The riffs on all the intros played on the radio was funny at first, but just lame, lame, LAME! the next five times. And then there was the whole deal with Peter’s mom (Phyllis Diller) romancing Tom Tucker, the local news guy. Turns out Peter’s real dad wasn’t that nice, but Tom Tucker’s turning out to be a better father, and Peter instantly reverts to a 300-pound man-child who gets pushed on the swing, gets his scrapes kissed by Mommy and overdoses on sugar. But I bet the real Peter would have the same happen to him. The bit about Tom Tucker still talking like he’s on the newscast was a nice one, but it was the only one in a show that is starting to show some weakness.

On the other hand, “American Dad” actually had a plot with a message, and handled it very well. Steve was worried that his son was dating a fat girl, and realizes that he, too, is on the train to Fatland when he checks himself out in the mirror. So he hires a tattooed and buff guy off the street, who makes him exercise to “Let’s Hear It For the Boy” and use babies as dumbbells. Ho ho.

There was a nice message about society’s obsession with being thin. The Smith family didn’t want Steve to be real thin; they liked him the way he was. And Roger the Alien actually liked the fat girl, doing everything he could to get her attention.

Just because it’s animation doesn’t mean it has to be one crude joke after another. Why do you think “The Simpsons” (which had a very good Bart-centric episode) has lasted for 237 seasons? Or something like that.

The fact that “American Dad” and “Family Guy” haven’t been consistent is baffling. They’re both created and produced by Seth McFarlane, who obviously can’t handle two shows at once. Hey, Seth—you’re no J.J. Abrams. Heck, J.J. couldn’t keep “Alias” afloat for very long while he got “Lost” off the ground. And now he’s got “Six Degrees” premiering. I’m hoping both shows can last.