Clutch slipping, intermittent

1999 peterbilt 379 Cummins N14 celect
Super 10 millions of miles....

Hello ,
My clutch slips at random. Usually when I'm into the throttle at speed. Tonight it got worse. I'm only grossing about 41,000 lbs. But tonight I'm bucking the wind going west in Texas. I'm wondering if a rear main seal leak can cause this? I grossed 78,900 last week Texas back to Ohio and only noticed it slip a couple of times. Seems like tonight it worsened , like the wind was making it worse.. if that makes sense? It wouldn't slip at all last week upon taking off loaded or torqing up through the gears. Thanks in advance and Happy New Year!

 

Thanks for the reply Chubby Fly. After sleeping on it I'm thinking it's internal transmission itself. There is freeplay and the clutch was adjusted 3 months ago when I took over the truck. It's been randomly going into neutral in high gear at cruising speed, 61-70 mph. I'll be driving along and suddenly the engine will just tach out, which is scary because the engine isn't governed! I'm going to call a shop here in Tyre and get a fresh opinion. Thanks

 

Update - At Werner Diesel Service in Tye, Tx . He's saying it's not clutch but likely moisture has gummed up the lines and solenoids in the transmission. On going air dryer issues likely to be the original cause. I have been draining a LOT of water from the air tanks since taking the truck.

 

Thanks AmodelCat, I posted my update before I noticed your response. What do you think about what they're telling me? The stick isn't moving out of gear. The clutch will pull the truck against the park brake. It just like go into neutral in high gear at cruising speed and the engine tachs out..... like it's hung between splits?

 

I don't normally use cruise control but it does it on the cruise and foot. The transmission is supposed to be a Top 2 automatic. I apologize for leaving out details. My mind goes quicker than my spelling and grammar abilities. There's nothing in cab or on the transmission lever indicating it's a Top 2... I'm wondering if it's been swapped for a straight super 10 and the ECM is not correct. .? Thanks all, I know it's difficult without the details but everyone's input is greatly appreciated.