EVERYTIME YOU THINK you've heard the last of Paul Cummings, he turns up again, a little more grizzled, a little more lined, perhaps a little more cynical and certainly older. A distance runner of immense talent, he has tended to disappear for years at a time on and off during the past two decades, only to reappear in the winner's circle again. In the meantime he waged his private battles with allergies and illness. He was a high-performance automobile that, when tuned precisely, was unbeatable, but getting that engine tuned was difficult at best.

When Cummings disappeared quietly again in 1987, it seemed like the end of the line. He is, after all, four months shy of his 39th birthday, and even aside from his delicate health, he possesses the kind of luck that can cause a truck to drop a load of pipes on him during a training run.For five years Cummings went without a victory of any significance. Then last week he won the Salt Lake City Classic 10K road race, and suddenly he was back from nowhere again. Where have you been? This weekend he will run a 10,000-meter track race in San Francisco in an 11th-hour bid to qualify for this month's Olympic Trials.

"I want to see what I can do," he says simply.

But it's more than that. By now Cummings knows what he can do. There have been American records, world records, national championships, the Olympic Games, 3:54 miles, 2:12 marathons. In his prime, in full health, perhaps no one could have beaten him. But the fragile health has made for a roller coaster career.

A few months after winning the mile at the 1974 NCAA championship for BYU, he was introduced to allergies while running past a lawn mower during a training run. Life has never been the same again. His asthma was so bad that he'd show up at the track some days and couldn't run two hard laps. He finished seventh in the NCAA meet that spring.

Cummings continued to believe in his talent anyway. He held odd jobs bagging groceries, cleaning carpets, and working as a construction gofer and groundskeeper for a few years, trying to hang on to his running career and support a family at a time when amateur rules forbade athletes from earning money for their sport. For years his training consisted entirely of running the seven miles to and from Geneva Steel mills, where he wielded a six-foot crow bar tearing down fiery hot furnaces. Graveyard shift, swing shift, day shift, it didn't matter; he always ran to and from the mills, night or day. He raced when he could. Once he worked an eight-hour shift at Geneva, rushed to the airport, flew to Philadelphia, won the Penn Relays mile the next day, caught a flight home and arrived in Provo just in time to work the swing shift.

Cummings and his family struggled to make ends meet. They were barely able to pay the bills. Then came a layoff and the running boom and a change in the amateur rules and a medicine that controlled his allergies. He ran to riches; ran to a dream house in Orem; ran to the 1984 Olympic Games and lived happily ever after.

That should have been the end of the story, but it wasn't. The allergy medicine lost its magic, and Cummings lost his health. The prize money stopped coming. Shoe contracts dried up. He lost the house, filed for bankruptcy, took a real 9-to-5 job. He was back where he started. And something else: He was older.

His career has been in decline since 1987, but he's always trying to make comebacks. Every time he trains hard, he gets sick. But last winter his body began to work again. He trained hard and often and stayed healthy, and suddenly he was thinking of the Olympic Games. He was running down 90th South in Sandy one day last February when a load of pipes fell off a passing truck and struck his legs. That cost him 25 stitches, a bone infection and several weeks of training. His weight ballooned to 176 pounds - 40 over his race weight.

Three weeks before the Classic he resumed hard training again and won the race for the fourth time, with a time of 30:36. Given the altitude factor, that effort was probably worth a 29:36 at sea level. It will take a time of 28:38 to qualify for the Olympic Trials.

"I'm just a runner at heart," says Cummings. "I can't give it up. You always come back to the things you do best. Plus, I feel better when I run."

So the old practiced routine continues. He rises at 5:30 each morning to run with the family dog, Sparky. He runs again during his lunch hour. Last week he ran 175 miles.

If nothing else, there is the gnawing sense that he never reached his potential. He has only been teased by it. He looked unbeatable in 1984, running the fastest time in the world and winning the Olympic Trials with the greatest of ease. But at the recommendation of a doctor, he stopped taking his allergy medicine after the Trials, and he ran miserably in the Games. He is convinced he would have medaled had he continued with the medicine.

Now he's pushing 40, and it is unlikely he'll ever regain his old form. But Cummings runs on. "I still think it's not over," he says of his career.

Don't think you've heard the last of Paul Cummings.