What's slipping

[GSMNP125]

I need help with a problem. Here's what's happening. When I downshift and acceleration something starts to slip. The majority of the time the slippage is after a downshift from 5th to 4th to accelerate up a hill. Sometimes it slips when I accelerate without down shifting. It doesn't slip every time and if I ease off the accelerator it quits slipping. It started slipping during the Winter and seemed to quit this Summer. Don't know if that's a coincidence or not. I checked the transmission fluid and it was fine. What should I do next?
Thanks

 

[3rzrunner]

..............CLUTCH!!!!!!!!
it sounds like your clutch is going.... sorry for the news....

 

[Hyde802]

Slipping like RPMs going up and your not going faster? Sounds like a clutch to me too. Maybe try adjusting it first?

 

[KidVermicious]

What they said. Check your cable tightness and clutch fluid level, but it's probably the clutch. Once it starts slipping there's no telling when it'll decide to completely let go, so if you have to drive it, baby the hell out of it and get it fixed soon.