Snapper 85-87(?) 6 series Model 25066 starts fine, stalls in gear?

Single blade mower, 25 or 26" cut. Tecumsah engine. Manual start (no battery) Start up fine & mowed fine for about 30 minutes. Stalled when I tried to put in back in 1 out of neutral. Started right up again 1st pull, but stalled when tried to move into 1st gear. pushed it back to shed. After 20 or 30 minutes I checked no obstacles under deck, oil ok, etc. Started right up, went into 1st and was able to mow for 20 minutes or so. Stalled again! Pushed it back and started up push mower. After 20 minutes or so, tried Snapper again, started right up, moved into gear, mowed about 15 minutes, stalled again. Gave up and finished mower with push mower (Uugh!)

Spark plug is new & clean, fuel filter looks clean. Oil is fine. Blade appears to move clean not hitting or rubbing anywhere. Blade belt is loose, but this shouldn't affect moving into gear, should it? Any ideas on what's wrong?

thanks!