This is my first post so I'll try not to screw up. I think I've read every post on here about overheating and thermostat replacement. Here's my problem. My 99 XJ, 110,000 miles, can sit an idle forever and not run much over 210 but at speed and hard acceleration the temp gauge will jump to 240 (first orange line), sit there and then drop back to a little over 210 once I let off the gas. I just had the coolant flushed and I’m thinking it’s either a clogged radiator or a bad thermostat. If it's the radiator how hard is it to change? Any feedback would be greatly appreciated.