Last week while driving on the freeway the engine suddenly lost nearly half its power.

Traveling uphill to take the next offramp was considarably more difficult in terms of engine power. Before finding a place to properly pull over, I waited at a red light for a minute, and noticed that the engine barely would stay in idle. The car vibrated and was literally shaking to the point that I could hear there was something definatly wrong with the engine. According to the dashboard temperature guage, the engine still maintained a normal operating temperature, however I could see and smell smoke definately coming from my car. I quickly found a place and pulled the car over so I could open the hood, and find out what was going on. The smoke seemed to come from below-behind the water pump. Since then I have changed the oil and filter, as well as the spark plugs. There is no improvement. I am completely puzzeled about what could possibly be wrong. Also, it might be helpful for you to know that: there was no mention of engine problems by the ez-lube tech who performed the oil/filter change. However, I changed the spark plugs myself and noticed 2 of the old plugs were completely submerged in oil(I believe it was oil). When removing the old plugs, this fluid simply drained downward thru the spark plugs opening. When starting the engine after replacing all 4 plugs, my engine initially produced a large amount of smoke from the tailpipe lasting about 5 seconds. Smoke is no longer emitted from the tailpipe(only this one time). A friend mentioned that the head gasket may be blown. There is no sign of water mixing with oil when removing the oil resevour cap or dip stick. What do you think is causing this engine to perform this way. Any help would be greatly appreciated. Thank you very much.