1st off - take you truck to a garage and have them check it out to see why the oil pressure is low. You said you checked the oil level so it's not that.
That thing (your truck) has a "T" connector where the oil pressure sending unit connects to the engine block and those have been known to get plugged up with crud and mess up your oil pressure readings. It may just need the "T" connector cleaned out but it could be something more serious (maybe the oil pump..).
As for the gauges are going crazy - that's a typical Chevy problem. Happens a LOT. The only answer I've ever gotten is to replace the entire instrument cluster.