My Dyson DC07 cuts out every few minutes and the filter is always dirty?

My Dyson DC07 cuts out every few minutes,suction is not what it was either,not swirling.Husband has dismantled it a couple of times and cleaned out cyclone with air jet. Only helps briefly.Filter is always filthy,have now bought another to have one in the machine and another drying.Was reading a previous string and some people mention another filter.Where is it and how do I get to it?Also did any one find a spare thermal cut out switch and was it hard to fit. Thank you.