Oil pressure gauge fluctuates

I have a 2000 GMC Savana conversion van. The oil and filter were just replaced 1000 miles ago. The oil level is full, but....While driving, the oil pressure gauge remains around 45-55. When I begin to slow down below 30 mph, the oil pressure gauge slowly begins to drop. At idle, it drops to 20. It immediately goes back up to 45-55 as soon as I press the accelerator. The van is NOT overheating. What do you think this could be (oil pump, sending unit, ...)?