Oil Starvation problem

Hi - had terrible trouble with our Citroen over the summer. Loss of power, gear problems, blowers stopped working, pipes blocked so leak in driver footwell.... We took it to Citroen Leicester who suggested a new gear box for the gear problem costing about £3k! No thanks! Found an automatic specialist who told us about an oil starvation problem with this production run of C4s - we found out the car needs an oil change every few months.... So with a new turbo something, oil change and filter the car was working like a dream until tonight when I was driving home, it wouldn't go into 6th gear, kept revving, dropping down a gear and no power... I'm guessing it's ready for another oil change but f any one has any other ideas please let me know because it's becoming a right pain in the ar*e! Thanks