I have a 79 N5 with what is supposed to be a rebuilt hydro and it will do the same thing.I don't know if this is normal or not.If you shift down a gear it will pull just fine.If you post this message on the general forum you might get a better answer.
I have an 87 l3 with 4 speed hydro that was bot new. Mine has done this since new. I don't think this is a problem, just the pump bypassing because of too tall a gear. (built in safe guard to protect hydro.)