My 99 chevy blazer is getting intermittent fuel

I just replaced my fuel pump about 4 months ago and now my blazer is getting very bad gas mileage. I fill up and right at 1/2 tank I've only gone 100 or so miles. Also, I'll be driving and the oil pressure goes up as I step on the gas and then goes down when I step on the break. My blazer has also been dying on me recently while I'm driving. It'll turn over but it's not getting fuel. I'll wait about 5 - 10 mins and then try again and it'll start but only to die about 5 miles down the road again. I just tuned it up about a month ago (new spark plugs/wires, dist. cap & rotor, new air filter, new oil filter, etc).

I read in my Haynes book that if the oil pressure is not is below the specified pressure level that I should check the oil pressure switch/circuit because it'll act as a fuel pressure cut off device. Is this true? If so, where the heck is my oil pressure switch? For some reason the book doesn't say where it's at.

I would appreciate any thoughts. Thanks!!