Intermittant Lack Of Power

Hi guys, I hope you can help me with a little problem. I have a 1.9tdi Octavia 2003 ASV engine and sometimes while driving along I will find I cannot accelerate, it will keep driving but no pull. It sometimes stays like that all day but sometimes will pull like a train next time it is driven. It doesnt seem to have any pattern, warm days, cold days, sometimes it can be weeks before it happens and sometimes it will happen two days in a row.

I am getting a little hacked off with it as I was driving up a hill at the weekend and was in second gear and struggling to get up it with a car behind me!!!!