I recently bought a 2008 Manual JKU. First manual I've owned, and a recent issue came up about a week ago. I can takeoff in 1st gear fine and then shift into 2nd fine, but if I'm on a hill or have any decent acceleration/torque going, right after I shift into 3rd the RPMs jump. I shift right around 2k rpms, and once I get into 3rd/4th it feels like the clutch will slip, causing rpms to jump to 3k+, and after about a second they'll go back to normal(they stay high if going up a steep hill). If I'm going downhill or gently accelerating on level ground it's perfectly fine, and 1st and 2nd are also always fine. I thought this would just mean I had to replace the clutch, but there's no weird noise, no clunky shifting, nor a sticky clutch pedal. The only weird thing I've never had with any other manuals is that all the engagement of the clutch pedal happens within the first 20% of pressing it in. I can shift normally with the pedal pressed only 30% in. Maybe that has something to do with it. Let me know if more description is needed or if there's another forum that addresses this issue. Just couldn't find the exact issue with a solution. Let me know, any help is appreciated : )

 

That sounds like clutch plate slipping due to wear or weak pressure plate. Either way it will get worse until you can't hardly move. Just a maintenance component of clutches and who drove them previously - IE in my 54 years of driving mostly manuals I have only ever replaced one clutch from wear and use. And that was towing a trailer with a 5.9 Cummins Dodge one night in 6th gear I put my foot down and she sounded like an automatic.

 

Agree, description you gave sounds like clutch worn / pressure plate weak.. either way replacement is only cure.. You might check the hydraulic slave cylinder before pulling it down to replace the clutch and pressure plate. BTW the test for a slipping worn clutch when performed provides the results you described..