I have posted this in mutiple areas, but no expert responses, so I decided to try here in this section.....

So I did a couple 4th gear pulls on the highway and once I reached 4000 rpm the rpms stopped climbing like the belt was slipping, lack of fuel, or something. I know I have some clutch slippage when I WOTin 5th gear, but also get some vibes in 5th when I get into it (don't know if that is a symptom, normal, or something entirely different). Any insights?