After my car has been running for awhile(30+ miles) then it gets hard to accelerate. The engine will rev up and down with no increase in speed even though I am still pressing the gas pedal the same amount. If I keep driving it will get harder to drive. Then there is a burning rubber smell. There is a little wobble to one of the bottom pulleys. When it is first started the car will idle high and the RPM's will go up and down. After it warms up a little it idles fine.

What could be causing this?

 

I forgot to add it runs fine in the A.M. 30 miles to take my significant other to work and go back home. It's the evening time that the problems start.

 

Possible misfire -- Have you inspected the plugs, wires, cap, and rotor? Any CEL codes?

Also, is the transmission (auto) or clutch (manual) possible slipping?

 

I'll have my significant other check it out. CEL codes? No check engine lights come on or anything. I don't know if they work or not. We just bought it and I think it's been modified some. I'm not too mechanical but it does have a giant silver intake thing. lol. It also used to be an automatic but it's a manual now. With a weird tiny shifter. I think the clutch is OK. It's about a year old. What would the symptoms of slipping be?

 

Ok. Three of them are covered in oil, and one is dry but has red dust in there. We weren't able to take them out because we didn't realize they were in there so far. My brother said we need to put a new valve cover gasket in as well as change everything you said to check.