How is you coolant levels? I'm curious whether your coolant maybe leaking out. Would explain why your temp shot up because not enough in there and then dropped down because none was in there to gauge. If I'm not mistaken the temp gauge measures the temp. of the coolant, if there is no coolant, there will be no temp.
To answer the question up top about the o-ring, call you local AC Delco dealer for a PN 217-461. Look to spend about $15, it comes with another O-ring which I believe is for the fuel return side. Believe me, us Delco distributors aren't making much on those O'rings, but Delco themselves have to be making a killing.