Are you using the low side of 3rd? If so, try the high side of 2nd under the same load. That may tell you if the PD is slipping in the low range. The "feel" of the PD when it snaps into either high or low is also a good indicator. Another thing is to try a PTO load, such as a grinder, to see if the main clutch is slipping by lugging it down. (better yet if a dynamometer is available). Of course, dont do this for very long. Other factors: how long main clutch has been in tractor; service of PD (if any); frequency of those heavy loads; & hours on tractor.