Your engine has sticking exhaust valves. It's a very common issue on this, and your description of the symptoms is EXACTLY what happens. It's fine on start up but loses power after a mile or so. It eventually clears and it's fine until you start it from cold next time. A good rag up the road cures it for a few days but then it's back. The fault code stored is P0300 - Random Multiple Misfire, because that's exactly what it is.

You'll need to remove the cylinder head and remove the valves. The valve guides will need to be reamered and the exhaust valves replaced with new modified ones. You should be able re-use the inlet valves but it would be worth re-facing and re-lapping them. I appreciate this probably isn't a DIY job for you so you will need to go to a garage or engine machinist, preferably one that has previous experience of this problem.

Once this repair has been carried out this fault should be cured.