GRAND RAPIDS, Mich. (AP) _ Plummeting pump prices are just confusion-as-usual for service station operator Jay Ysseldyke, who's spent 14 years in a business where the price of customer loyalty is often just a penny a gallon.

Ysseldyke, whose Shell station has attendants actually pumping gasoline, has learned to live with the literal ups and downs of his trade.

''Sometimes it's hard on my gut. Prices go up and they go down,'' said the 44-year-old Ysseldyke. ''We've been through the shortage and now we're in the oversupply.

''You just try to figure it all out and go with the flow.''

Ysseldyke was selling regular gasoline for 80.9 cents Monday. But the convenience store across the street - the one with the lines of cars at the pumps - was selling it for 78.9 cents. And the doughnut shop to his immediate east was filling tanks for 79.9 cents a gallon.

He says the competition isn't exactly killing him, but keeping up with the independents and discount retailers isn't easy.

''We are struggling a little bit because of the independents,'' he said. ''We try to stay within a penny of them, but that's hard to do right now.''

The independents, however, are more concerned with one-upping each other. Just down the road from Ysseldyke's operation on Grand Rapids' 28th Street, a congested commercial strip lined with malls, chain stores and fast-food eateries, is Four-Star gas.

Chuck Kelley has seen five price changes since he started working at Four- Star. He started working there three weeks ago.

''The lines are pretty constant,'' he said of the station, where regular was 78.9 cents a gallon Monday. ''The customers love it that they can fill their cars up for less. They can't believe the gallons on the meter are higher than the dollars.''

That suits Julie Zeilstra, a 21-year-old student at Calvin College who drives around town in a thirsty 1972 Oldsmobile Cutlass.

Zeilstra says it costs her about $15 to fill up the old guzzler. A few months ago, before the latest price slide, she figures she could have topped off her tank for $20.

''I'm not really sure, 'cause I'd never had the money to fill it up very often before,'' said Zeilstra, of Oak Brook, Ill.

Like just about everybody else, Kelley doesn't know where it will end.

''We're going down another penny tomorrow,'' he said.

When that happens, he expects the competition at MSI, a few blocks away, to follow suit.

And frankly, MSI employee Bob Clark is beginning to get a little annoyed.

''We came down a penny today and we'll probably drop another penny tomorrow,'' said Clark. ''I've got to climb up that ladder and change the numbers every day.''