I agree that it sounds like there is an issue with your clutch. I had a similar situation this spring when I purchased a used 448 with a 48" deck.
The deck was not installed on the tractor when I got it and when I hooked it up I found that the clutch had been taken apart and not assembled correctly. It took some time and the studying the manual to get it back together and working properly. Maybe it is just coincidence, but it seems like previous owners choose to sell when they can't or won't repair a failing clutch. Take heart, by taking the time to learn how to repair it correctly the results will be worth it.
I found the manual I needed on-line as a downloadable PDF file, but I don't recall where. I have the file at the office if you want a copy of the file let me know. I can tell you that you will need to take some time to get the clutch repaired correctly. Should you need them the parts are available from a number of sources. You will find references to two styles of clutches and you most likely have the later version that is adjusted with a torque wrench and does not require different sizes of shimming for adjustment. There are shims in the clutch stack and you will want to pay very close attention to the order that things come off so that you can get them back together correctly. You may want to sketch or photograph the assembly as you take it apart.
You will likely need to get a large, thin headed open end wrench to hold the large nut in the tight access area. There is also a fork with two small friction pads that contact the clutch plate and are not shown in the service manual I had. It is correct to be there and it is a brake that stops the deck rotation when the clutch lever is disengaged.
While things are apart inspect the deck spindles and bearings. I found that my deck needed new spindle bearings, new idler bearing/pulleys, and a new deck belt. I also found that the existing blades were not straight and replaced them too. Though the original owner may not have been willing to make the repairs needed to the 448, I now have a very nice mowing tractor for my 2.5 acre yard.
Good luck and enjoy your new (used) tractor!