Nissan Serena 1997 Auto 2.0(white smoke, timing unconstant, dies when start to drive)

the car just got out from a workshop about a month doing top overhaul.but still got problem
1.easy to start
2.not smooth like 3cylinders only work
3.white smoke came out from the exhaust
4.no confidence to drive it on the road
5.dies when change to drive mode without pushing the fuel paddle.
6.timing is not constant and mostly getting dies.

-pressure test already done and no leaking
-head gasket already changed
-coolent does not decreased and normal.
-engine oil leaking just a bit and at an angle of the head.
-tuning havent done yet.
-and the mechanics dissammbled the timing chain without marking it first.

please give me some recommendation to solve this problem.

ur help would be appreciated.thank you