CHUNKYMASTER

hi ive got a chrysler grand voyager 55 plate 2.8 diesel i have a problem with the temperature guage i can sit on the motorway at 70mph for hours and the temperature guage goes no further than half but as soon as i put the engine under load (put my foot down)or go up a lond step hill the temperature guage goes straight into the red engine cuts power until the tem guage comes out the red back to crusing and it goes back down to half please please can someone give me some ideas what could be wrong thanks