renault traffic 06 problem?

hi my 06 renault traffic has just had new head and injectors now runs for about 6 hrs then sstarts jumping around and goes flat no oomph till it hits 2000 to 2000 revs.1st and 2nd gear near imposossible.fitted new sensor ran for a day same thing again .it feels like its not getting fuel .it will sometimes puff black smoke as if blocked then run ok for a few minuetes then same again .when tested this evening it said exhaust sensor problem.when tested again 20mins later no problem ,maybe engine was cooling down by then any help would be brill mechanic is at wits end and tottally naffed of with my van .