Needle on temp gauge still going up and down after changing thermostat and sensor?

So a few weeks ago I posted a question about my needle on temp gauge going up and down. I was given a couple of similar answers about changing either the thermostat or sensor...I changed both and my needle was stable for about a week... then I decided to go 2 bakersfield 2 visit family and before I could even get out of long beach, my needle all of a suddebn started going crazy again. The radiator is full and the needle only moves when I accelerate over 60mph or go up the slightest hill. Now mind u, driving up the 5 to bakersfield is mostly uphill, so by the time I reach the top of the hill my needle is at the top of the temp gauge, red hot..I pull over and open the radiator cap and the water is boiling, which means the engine is actually overheating and I'm not getting false readings. But mysteriously all this week the needle hasn't moved a milimeter... could it be the radiator itself or the catalytic converter since the check engine light is on because of the cat(I had the car scanned and the code for the cat came up)? Now I had the cat and all its components and the radiator replaced a little over a year ago due to an accident..... please help! I've run out of ideas!