On a day when it should have been detecting drug smugglers, the Customs Service's $20 million flying radar platform was towed to its hangar for repairs.

The radar dome atop the four-engine P3 Orion had stopped spinning in flight for the third time in a week this month. Radar specialists inside couldn't produce those snazzy color-coded displays on their computer screens, designating friendly and suspect planes more than 200 miles away.As the plane inched down the flight line to Customs' spanking new $4.5 million hangar, the agency's war against airborne and seagoing drug smugglers also slowed down.

For the Customs Service air branch generally, and the surveillance unit based here on the Gulf of Mexico, this can be the best and worst of times.

Less than a decade ago, Customs fliers felt like World War I aviators, bouncing around in light single-engine planes, using binoculars to spot the enemy.

But Rep. Glenn English, D-Okla., and Sen. Dennis DeConcini, D-Ariz., wanted a real Customs air force, and while many federal agencies retrenched, the air interdiction program shot up like an adolescent child.

There are 88 Customs airplanes now, including detection aircraft, trackers, interceptors and helicopters carrying officers ready to swoop down and make an arrest.

The program that employed 153 people in 1981 and cost $27.3 million now has a $165 million budget and 811 employees.

But expansion has brought severe growing pains as equipment has broken down, sometimes because it is too old and sometimes because it is too new.

Failures have plagued all seven airplanes here at Corpus Christi, where the mission is the weakest link in stopping drug smuggling - detecting smugglers' planes and boats and sorting them from the pack.

"We're looking for one little guy among hundreds," said Stanley Adams, an air branch chief and pilot in Corpus Christi. "We've caught most of the idiots. Now we're up against the smart ones. They do have the edge now, but it's a start."

At the top of the detection pyramid is the new P3 airborne early warning flying radar platform. Lockheed, which designed the plane for anti-submarine warfare and detection, modified it to include the 24-foot spinning radar dome atop the fuselage.

Tied to a General Electric radar used by the military, the plane has capabilities similar to an Air Force Airborne Warning and Control (AWACS) aircraft. The P3 has done well when working, Adams said, and in one crucial test near Colombia, it spotted virtually all aircraft taking off from a known drug-producing region.

But during a week-long visit by a reporter, the dome would stop spinning when the plane's speed reached 200 knots, and it had to be taken out of service while uneasy Lockheed experts tried to find the problem.

The early warning radar isn't the only problem plaguing the surveillance unit.

Crewmen said the main radar in four P3s used for detection and long-range tracking of smugglers has failed often, an event witnessed by a reporter.

The radar was built for an F-15 "air superiority" plane, an Air Force jet that stays in the air an average of one hour and 20 minutes. But during P3 flights of six and even 12 hours, the radar's guidance system goes haywire, giving operators false readings of planes and boats traveling 800 miles an hour.

Customs crews said that in an intense 10-day mission in the Caribbean earlier this year, the guidance system on one P3 was not functioning about three-quarters of the time, even though the equipment was replaced twice during the operation.