When it is good, Phil Johnson and Omri Schein’s spoof of upbeat, life-affirming Christmas shows is very good. But when it is bad, and it is too often, the the writing flags, the comedy’s too forced (or non-existent), and performances can’t salvage leaden scenes.

Christmas Spirit, who wears a silver-spiked fright wig and looks like a lounge lizard-version of Count Dracula, is on the outs with the Powers That Are. Even though it’s “the most wonderful time of the year,” the world is losing happiness. The Joy-o-Meter tells us so.

One reason, we learn: “Christmas hasn’t been the same since Andy Williams died.” Another: Bob Filner brought it down.

Spirit, whose diet consists mostly of bug-larvae, must create a “holiday spectacular” to express the full meaning of the season for all religions — “Chriswanzukah”? — and everyone else.

Rather than do it himself, Spirit orders the “bug-ish” Horatio the Humbug to do the job. Later, “Christmas Carol” Horowitz, a Jew fascinated by things yuletide, joins in. If they can’t create the show, Spirit will eat Horatio’s larval offspring. In the 85-minute, intermissionless piece, sketches portray various failures to achieve the spectacular.

I think that, over the years, Phil Johnson has taught us how to appreciate his work. At first it was: who IS this flamboyant, miles over-the-top whacko? Now it’s: Phil’s doing a show? I’m going!

Johnson plays Christmas Spirit and several others. The first is the most fascinating, especially for those of a certain age: Bandy Billiams, a soft-spoken, hyper-sensitive Andy Williams and his TV show, “Christmas in Sweaters.” Billiams is so emotionally fragile he can barely move (a touching creation amid the parody).

Johnson’s a hoot as It’s A Wonderful Life's George Bailey about to jump off the bridge. As he wonders if he’s meant anything, one of Satan’s minions talks him into making the leap. And he’s a scream as the baby Jesus, in a morning coat and swaddling who, at one point, whispers to the Virgin Mary, “Mama, change me later.”

The skits that don’t work die early. Debbie David’s “Christmas Carol” Horowitz does a cooking show — how to make a fruitcake — while slugging down enough robust slurps of Jack Daniels to plaster her in seconds (but that don’t even phase her until the end). The bit is never funny, and the Jewish humor — at least to this 3/4ths Goy’s ears (my grandmother, Marguerite Rosenberg, was Jewish) — verges on the anti-Semitic.

Omri Schein and James Olmstad’s music and lyrics are pretty good: Broadway-style tunes, accompanied by Patrick Marion’s piano and performed with Jill Gorrie’s snappy choreography. But Schein, wearing Renetta Lloyd’s inventive beige bug-suit, makes Horatio so one-note he’s always predictable, which kills potential laughter.

Sarah Errington shines throughout. She plays everything from a Holiday Pixie to Satan’s intern on Bedford Falls bridge, and she sings and tap-dances up a storm.

But neither she nor Johnson’s at-times-brilliant antics raise the Joy-o-Meter for long.