Originally Posted by 
AK83
				
			 
			On the topic of the cooler. 
I would suggest the simplest things first(and then test again).
Have you visually checked and confirmed that the radiators are not blocked in any way.
TD5 has a lot of radiators up front.
AC condenser. Do you have the AC on whilst this heating up is occurring?, if so have you tried/tested with AC off? Does it do the same thing?
if AC condenser partially blocked. Dirt, crusted up silt/mud, etc. it will affect the efficiency of the AC system(not so cold) and block air through to the trans cooler
Hard to see properly, but the cheapest(theoretically zero cost), and time consuming method would be to pull out radiator, give it a good flush through with garden hose whilst out, and do same for trans cooler. 
Have you checked the level/colour of the trans fluid?
Temp sensor: my experience(in my tdi, but very similar temps sensor) mine came on, whilst very light duty driving(up hill). Slight panic(wasn't expecting it) checked trans level, but everything was hot, and fluid level is done when cold, idling in drive.
Drove gently home light on, off, on .. for a good while.
I have an IR temp gun, and trans sump was saying about 60-ish °C! Stopped panicking and thinking maybe an earth.
Next day from stone cold(trans fluid level good) ign on, trans temp light came on(stone cold) started up and it went out. It would come on randomly, maybe stay on, maybe off in a few secs. Ran like this until the new sensor arrived, and with new sensor fitted, all back to normal. 
I'd reckon your sensor may be OK.
So check fluid level.
Radiators clean
Then assess the spending $s if everything else is normal.