I own a 2005 S60 Volvo 5 cyl 2.4L (no turbo), manual 5-spd with just under 110,000 miles. I’ve been driving this car less than 3,000 to 5,000 miles per year for the past 6 years. The problem I’m currently having is a stiff clutch pedal after I drive my car for a short period (10 minutes+) at highway speeds above 45 mph. The longer I drive at highway speed the stiffer the clutch pedal becomes until the stiffness becomes quite significant. The clutch pedal is just fine and not stiff at all if I do in-town only driving and stay under 45 mph. And even if I do highway driving for a short while the clutch pedal stiffness dissipates not long after (20-30 minutes) I continue driving on city streets at a lower speed. Also the clutch pedal stiffness lessens after being parked for several hours and it completely goes away after being parked overnight. Interestingly, this problem has been on-going for the last 2 years starting at about 97,000 miles, although it's never caused any other problems with the transmission or anything else; even if I do a few hours of highway driving with a very stiff clutch pedal. There's no problem shifting the manual transmission, even with a stiff clutch pedal, as shifting is as certain and smooth as when the car was new. I did notice when this problem started 2-years ago it took over 3 hours of constant highway driving around 65 mph or higher for the clutch pedal to become stiff, but as time has progressed the clutch pedal stiffness now occurs much sooner. I've never replaced the original clutch and the transmission filter and fluid were drained and replaced at 82,330 miles. I want to do some long driving trips this summer, but don't want to damage the clutch or transmission in the process and create an expensive problem to fix. I would greatly appreciate it if someone has any idea they could share with me why my clutch pedal is doing this and what may need to be done to diagnose and cure the problem, and avoid making this an expensive or disastrous problem in the future. Any help would be appreciated.