The oil pressure gauge on my 1991 GMC Sonoma fluctuates between reading way too high and reading normal.?

Sometimes it makes quick jumps between the middle of the gauge and the top, other times is sits around 60 for a while, then jumps to the top of the gauge. Other times it pegs for long periods of time. Sometimes it appears to be connected to acceleration, but it does this at idle, in town and on the highway, although generally it doesn't move around much at idle. What does this mean?