Hi thanks for responding so fast as usual
I have found the problem and it is unrelated to mod_cache and mod_proxy. The problem is the bandwidth limiting in lighttpd is extremely poorly implemented.
If using a backend such as linux-sendfile as much of the file is sent as possible in one pass. This bypasses the bandwidth limiting function altogether. Instead on the next pass and subsequent passes, lighty stops sending anything out until the bandwidth limit is satisfied. this can mean very long pauses resulting in no data being transmitted and the clients disconnecting and reconnecting.
It is a very undesirable system
Chris