Oil pressure gauge

In April I bought a Dodge RAM 1500 SLT v8 with
142,000 miles on it then two weeks later drove it
from Louisiana to Indiana and back no problems
then a week later I  drive it to Texas and back to
Louisiana no problems. So I go to the Dodge
dealership to get my oil changed then a week later
my oil pressure  gauge drops while I'm driving. It
drops then goes back to normal but when I drove
through a big puddle of water the gauge went back
to normal and drove for about a week with out
dropping again. It dropped , I drove through more
water then it was fine again. The truck runs fine no
knocking or loud noises.  Help please , thanx.