This doesn't sound like clutch problems at all. It sounds more like the engine is starved - not getting the fuel that it needs under load. Like previously stated - your tach should climb if the clutch is slipping - and it sounds like yours is dropping. The car is almost stalling out. A clutch that slips in first will slip in every gear when under load - not just first. The humidity may in some way be fooling the MAF sensor, but you'd need a more informed opinion on this than mine. But I seriously doubt that it's related to the clutch.
Look for links like this - http://www.ehow.com/about_5382536_ma...-symptoms.html
I'd try cleaning it first, as per the instructions here. That could be your very inexpensive fix.