Leg numbness and pain when working out on erg

Hello all. This is my first time here and my first post. Hoping to get some insight on a problem I've been having for the past year or so.

I have a Concept 2 Rower (often called an erg) that I've been using for more than 20 years. There have been times when I put it away, for example about 12 years ago when I had arthritis in both hips to the point where I couldn't bend forward enough reach the handle when it was in its cradle. After successful bilateral hip resurfacing I took it up again in 2011. I'm 65 years old and not as strong as I was 20 years ago so, now may main focus is maximizing time and distance at a moderate pace, with occasional HIIT.

The problem that I've been having is that sometime around 20 to 30 minutes into my workout (my normal 3x per week is about 70 minutes) my right let starts getting numb. Also, my right buttock starts to hurt. I have to stop for a few seconds every five minutes and stretch to my toes while still in the seat. This seems to help somewhat but I have to keep repeating, and it usually gets to the point by 50 or 60 minutes that I have to stop, get up, and walk around, which puts things back to normal. I often get to the point where my right foot is so numb that I have to wait a while before I can stand on it with balance. This never happens except on the erg. I have tried putting the standard Concept 2 pad on the seat, but this doesn't seem to help.

I'm assuming that this may have something to do with my sciatic nerve but the symptoms don't really match any of the conditions I've seen on the internet (sciatica, piriformis syndrome), especially since this does not happen except when seated on the erg.

Does anybody have any suggestions as to what specifically might be causing this, and / or what I might do to relieve it without giving up my workout?

Thanks in advance for any advice.