Still better than the explorer post..

 

I had a similar issue, one day I was driving fine, and over the course of a week it lost more and more of its power til i was not sure if it was safe to go onto the road. One big symptom of mine was a jumpy timing mark. Turns out the problem was a bad crankshaft timing gear. They go bad a lot if shops change the timing belt because they don't tighten the big crank bolt enough (found mine at 20fp when it should be 180) so my gear was half sheared and caused terrible timing and lost power. Check your timing to see if you have this issue, go for a drive and check if its moved at all, if it has I would go ahead and look at the crank timing gear. Mine easily spins wheels on the dirt not and would have no issue keeping 80-90 if I wanted... Hope this helps

 

I have a supercharged 3.4 now, but my 3.0/5-speed never felt gutless or struggled like you describe. Something is definitely wrong and you should start with a maintenance sweep. I'm not saying dump money unnecessarily, just make sure it isn't something simple like worn or incorrectly gapped plugs, incorrect timing or a clogged cat.