I have an 03 civic lx and it has a new radiator, hoses, and t-stat. Leak dye test at radiator has been inconclusive for head gasket leak. Runs fine for about 2 weeks then gauge starts to spike. Stop for about 10 minutes and gauge reads normal again. Have not noticed any fluid loss. Fans both come on normally having recently replaced the cooling fan relay. Does anyone know if it is a bad temperature sending unit or is there likely a crack in the cylinder head or exhaust manifold?

 

Sound like headgasket. You might need to get some ARP headstuds and a new headgasket so this doesn't happen again. Mine is doing the same thing. If you change the t-stat and fan relay than the headgasket is the next optioin