Im not sure what the problem is but, when i start off in first i press the gas let of the clutch slowly. my foot could be completly off the clutch and the car will still be reving. When i put it in 2nd and then i mash the gas the car revs first then the car goes.
the thing is slipping for a reason. either the cable is all out of adjustment (someone put a ton of preload on it. if you get under and can tug the cable at the bellhousing and there is slack, preload is likely not an issue).
the disk could be saturated in fluid or the pressure plate fingers let go. there are other reasons, but those are some of the biggies.