Two things I think it could be.
Has it been like this for long or did it just start being cranky one morning? Is there any difference when cabin pressure becomes regulated (aka...warmed up)
I just took my gauges out and apart on my 95 SSEi because I was getting freaky oil pressure behavior. Also believe my boost gauge was having odd behavior as well...but I've had the car less than a week and dunno what the heck the boost gauge is supposed to do at idle etc. Got the stomped to the floor part down
There'* some stuff in techinfo about it if you search on "gauges".
Secondly I'd suspect the sending unit that is supposed to be located down by the oil filter. I bought a new one right before I got the car and haven't installed because it didn't seem bad. Usually the senders will be all the time bad or all good. Mines intermittent...probably the altitude I was cruising.
Need to get my front exhaust manifold back on then will be able to share more info as to whether I'll be changing the sending unit or if it was only a corroded pins on my gauge cluster.