Juddering

I have mentioned before that occassionaly when in second gear about 2.5k revs my car starts juddering really harshly that the car feels like it wants to stop. I then have to normally put it in 3rd which means you loose all power.

This morning I was driving up a steep hill and it started happening it wasn't liking 1st as I was still moving at a bit of speed so had to go into 3rd and virtually crawl up the hill with no power (looking back I should have slowed down much more and then 1st might have been ok).

Does anyone know what this might be the car was up to temperature had been runing for about 25mins without any problems and then all of a sudden it just kicks in.

I always wondered if it was something to do with the fuel supply especially when cold but as it had been running for a while it's got me a bit stumped.