There are multiple things that could cause this behavior:
1) Check the EGR valve and see if it is sticking open. You could disconect the air hose that controls it, plugging the end of the air hose you disconnect. If the engine runs just fine without the EGR valve engaging, then this is the problem. You can also use a vacuum tester to see if the EGR valve opens when vacuum is applied and closes quickly when there is no vacuum.
2) I don't have my manual in front of me, but I think the manual mentioned some type of air control valve that could cause this problem.
3) It could be that there is a gasket leak somewhere in the engine block/intake manifold. When the engine is cold and the engine is running, spray carb cleaner along the sides of the engine. Careful, this stuff is flamible. If the engine speeds up from the spray, then you know you've got a gasket leak, and you can pinpoint it with the spray.
