1999 suburban temp guage freaking out

Ok virgin poster here so bare with me. I have a 99 burb and i had some hard start issues the other day. I pulled plugs and they were carbon fouled. so i also noticed the cap and wire need to be done again (did it last yearusing autozone parts) so i replaced all. The truck runs fine. EXCEPt that the temperature gague is now freakin out!!! i bought a new temp sending unit from ( i hate this word ... AUTOZONE) and this is the one on the drivers side of the block with the single wire and pigtail. hooked it up and it is worse than the one that was in there. it pegs the temp needle at 260. now the one that was in there was new 2 month ago and worked fine until the wife tried to start the thing sat and it wouldnt run. after the tune up the guage would read 240. I think the guage is FUBAR..I need answers. After 2 seperate sending units im leaning to a bad guage but why? Cranking an egine shouldn't kill a guage unless a voltage spike hit it and a tune up dang sure wont....by the way using an infrared thermometer the engine runs 180 with a new tstat and coolant temp sensor...