Hardly anyone would have predicted that the European Repertory Company, with its interesting but uneven track record of producing often obscure European texts, would have come up with one of the season's best and most durable productions. But it did: ERC's Agamemnon, which opened last September for a limited engagement, this week heads into a well-deserved open run. British playwright Steven Berkoff's update of Aeschylus' tragedy, about the Trojan War hero who's murdered by his vengeful wife, boasts impressively tight ensemble work under the direction of ERC cofounder Dale Goulding, a British expatriate who studied under Berkoff in London. Featuring acrobatic and well-executed choreography and an eerie, punkish visual scheme, the show is both playfully imaginative and chilling, recalling the fanciful, folklore-inspired productions of the Organic Theater, Godzilla Rainbow Troupe, and other companies that helped launch off-Loop theater in the late 60s and early 70s. The verse script, written during the Vietnam war, employs ironic contemporary slang but preserves, even heightens, Aeschylus' sense of gravity in describing slaughter and anarchy. Remarkably for a non-Equity production, the extension requires only one new cast member: Goulding himself, who's taking over the title role. European Repertory Company, Wellington Avenue United Church of Christ, Baird Hall, 615 W. Wellington (courtyard entrance); 248-0577. Saturday, December 16 and 30, 8:00. Then in an open run starting January 5: Fridays and Saturdays, 8:00. $15.