Need He;p.

1999 XJ8 sport Auto 63k. Engine stalls going down hill. 4 times in the last couple of weeks the engine has stalled on the overrun, going down a slight incline at between 30 & 50mph. No problems idling and is ticking over at about 625 revs. A couple of years ago I had a similar problem sharply accelerating, joining a motorway, but after taking foot off the accelerator, the engine would stall. From memory I had to clear out the left rocker breather pipe with a 6mm drill. The other suggestion was cleaning the butterfly flap of carbon on the intake. This is a slightly different problem. Any suggestions?