Gasoline prices bucked a summer trend and dipped a quarter of a cent a gallon during the last two weeks, according to a nationwide survey of more than 12,000 stations. The Lundberg Survey said the average price of all grades of gasoline offered at all types of service stations was 99.98 cents as of Friday, down from the June 10 average price of $1.0023 per gallon. The average price was down nearly a full cent from comparable June 1987 prices, according to the survey released Sunday. ``It is untypical for retail prices to be falling at this time of year,'' when the demand from vacationing motorists usually pulls up the price, said survey director Trilby Lundberg. She blamed the dip on competition between dealers and price instablity on the global oil market. The survey found that average prices at self-serve pumps as of Friday were: regular unleaded, 90.84 cents a gallon; premium unleaded, $1.056 a gallon; and regular leaded, 88.09 cents a gallon. Self-service pumps account for eight of every 10 gallons of gasoline sold. At full service pumps, the average prices were: regular unleaded, $1.1964; premium unleaded $1.2978; and regular leaded $1.1592.