I thought you wrote that you put a pressure gauge on it and had 45 psi. Why would you change the regulator or anything else in the fuel system ?

 

I put a gauge at the rail last weekend and it was 65psi at idle. That's why

 

The '88 came with a low-pressure pump in each tank, and a high-pressure pump along the frame rail. IIRC, they went to the single high-pressure pump in the tank in '90.

Perhaps check your fuel return line to ensure it isn't kinked or otherwise unable to return fuel back to tank...?

In the way-out-there category of possibilities - could the previous owner have replaced one or both of the low-pressure tank pumps with the later model year high pressure pump?