I have no experience of peugot diesels, only seat/vw diesels. But losing power at speed sounds like a classic MAF sensor problem (Mass Air Flow sensor found between the air filter and the turbo air intake).

Cold starting problems are likely to be faulty glowplugs, these usually start failing one at a time after 60,000 miles or so, and although not expensive can on some engines be a bit of a pig to change. They are easy to check with a multimeter, they usually fail open circuit, should be less than 1 ohm when cold.