After a head gasket job, van runs poor

I just had the head gaskets job done on my 95 villager. I had the mechanic also replace the timing belt,the water pump, the seals, the spark plugs and the thermostat. But I am very disappointed because it is running poorly. The timing is off I believe. Does anyone have any suggestions as to what could be the problem??? Please let me know.
Thanks

Mujier

Bring it back to the guy who did the job. He "F-ed" your mini van up. Bring it back to him and tell him to drive it. It could be alot of things. Like, Plugs aren't gaped, Broken wires going to Cap, Bad Ground, Bad Vacumm...

I did and it was the wrong plugs installed. Replaced the plugs and the van ran like a charm. But today a weak after the engine light keeps coming on and off and my wife said it drives heavy. I will be taking it back to the mechanic this afternoon.
Thanks for your reply.