The average price of regular self-serve unleaded gasoline in California slipped to $1.52 per gallon last week, down a little more than a penny from the week before. The average California gas price peaked at $1.53 per gallon on April 9, the Department of Energy said. The national average retail price for unleaded also eased slightly to $1.23 a gallon after hitting a 23-month high last week of $1.24, the weekly survey of 800 gas stations found. Prices have risen lately because of higher oil prices and increased demand, analysts have said. In California, refinery problems also contributed.