Ive got this strange problem with my Canon 24-105 lens (1.5 years old)
the other day when i tried to shoot video or stills on it thru the viewer the light was going in and out in the viewer and when i hit record or took a photo it seemed to shudder as if it would not connect to my 5d body sensors.

I change to my 50mm lens and it has worked fine - could the sensors on the 24-105 len need cleaned and if so how to i best do that? It seems to happen intermittently - last week I had a message come up on the viewer to clean the sensors

The sensor (there is only one) of the 5D cleans itself every time you switch the camera off and on. The problem you describe has probably nothing to do with dirt on the sensor, anyway.

When the sensor is really dirty, the automatic cleaning does not do the job anymore and you have to clean it manually. Use the special dust-free swabs to do so. They are currently available and if you follow the instructions it is not difficult nor dangerous for your sensor.

Edit: after reading your post again, I think you mean the contacts on the body, where the lens contacts fit. You can clean those with alcohol to degrease them. But I have used Canon bodies for years in a professional environment and I never had to do this. So probably one of the contacts is defective.

Possibly a common problem with the lens.

The 24-105 has had more than a few diaphram failures. You can see one occurr in a youtube video.

Faulty Canon Eos 5D + 24-105 / 4L IS USM Part 2 - YouTube (http://www.youtube.com/watch?v=oATVunUg7C0)

Canon 5D Mark II - Firmware 2.0.4 Bug / Malfunction - IRIS issue - Video 2 - YouTube (http://www.youtube.com/watch?v=TaOnN3aZO6E)