Speed variation

We have ADSL2 broadband. We're not a long way from the exchange (line attenuation OK) but our speed varies a lot. I ran about 20 speed tests over the last 2 days and download speeds vary between 12.4 and 7.6 Mbps, upload from 0.94 to 0.29 and ping mainly 16-24 but occasional jump to 140. Our ISP has changed the signal to noise ratio in the last 5 days but we still get this variation. I am thinking of getting BT to check the line but it's about £150 if they don't find a fault. Does anyone more experienced than me (not difficult) think that these variations suggest a fault or is this normal based on local traffic at different times of the day? Our ISP say that an engineer may test the line but find no fault if the line is OK during that particular 30 minutes or so. I'm not sure what to do.

I'd be grateful for any comments.

Without seeing the actual speed test results, and that is why the ones we run at http://www.thinkbroadband.com/speedtest store the graphs for sharing

and also knowing what modems sync, attenuation and noise margin figures are its hard to say a lot more.

Generally if the speeds are good between 7am and 2pm and start getting worse from 4pm you are looking at congestion (shared nature of internet) things should usually improve again around midnight