low pressure on hot only?

My Triton Opal is approx 8 years old and suddenly developed a problem that the pressure is really poor when on hot setting (anywhere between 8 & 10) setting on dial and yet the temp is only warm - not hot. I have cleaned the head, removed the inlet filter as it was rusty looking but nothing made a difference. The low pressure light does not come on - the micro switch seems fine and PCB joints all look ok. If i turn the temp dial to cold the pressure is really good.
Any ideas much appreciated?
Thanks Matt