[Client thread/ERROR]: Item entity xxxxx has no item?! causing lag/glitchy behaviour in SMP 1.8.1

Hi All, new to this and MC in general, I hope I'm putting this in the right place.

I am playing SMP on 1.8.1 with a friend, and I am having trouble everytime I activate doors using pressure pads, go through a portal, ride a horse etc. I lag out and sort of rubber band between different locations and my friend says I appear to be standing still. I get the following messages in my Game Output (see below), the times of which correlate to these laggy events.

I would be most grateful if any one has any suggestions /if this is really a bug or of any ways to fix this.

Cheers

[20:57:45] [Client thread/ERROR]: Item entity 45897 has no item?!
[20:57:45] [Client thread/ERROR]: Item entity 45897 has no item?!
[20:57:45] [Client thread/ERROR]: Item entity 45897 has no item?!
[20:57:45] [Client thread/ERROR]: Item entity 45897 has no item?!
[20:59:46] [Client thread/ERROR]: Item entity 56112 has no item?!
[21:12:53] [Client thread/ERROR]: Item entity 98369 has no item?!
etc.....