Problems with occasional overcharging

Hi. I have a gen-1 (NHW-11) 2000 Prius that occasionally continues charging when the battery has passed it's normal level, and if I don't completely switch off then restart, it will go over 100% and eventually throw a red triangle. During normal driving I will first notice it not shutting off when coasting or braking, then if I keep going it will start to rev the IC engine to spill power whenever I brake, and then even when stationary it may rev for a few moments. After the restart it will run very "efficiently" until it brings the battery down to normal, then life continues as if nothing happened. I get this perhaps 1x / 2 weeks. My car has done 160,000kms (100 K miles), and has had the battery serviced (bad cells replaced). This has been happening over the 3 years that I've owned it, both sides of the battery service. To me (a senior software eng) it seems like a program bug. My car is fairly early (serial# ~7000) so I wonder if there's a firmware update available? My wife's 2002 NHW11 doesn't exhibit this problem. My fuel consumption is pretty good - 50.2 miles/gal over mixed highway / city driving. My regular route is ~20 miles each way daily, all on flat roads (no hills).

Anyone have any ideas? Please read my post carefully before replying as I've thought about this quite a bit and have stated accurately what I see. Many thanks in advance.
-Ken