300SE Over-Charging??

[Cliff_W140]

I replaced the Battery/alternator in my 93 300SE in January. The car has not been driven much since that time because of the new W210. Last week I got her out to prepare her for the market block. The day after detailing her, I come out and the battery is completely dead. I charged it over night, and started it up to check the alternator. At idle it was 13.8, thats solid. I drove it around town all day with no trouble. The problem only occurs on the highway. After cruising for about ten minutes, the idiot light comes on, Radio/AC/Cruise control cut out and the Battery light flickers off and on until I park. It starts right back up, and seems fine around town at low speed.

My guess is either a short in the battery, or the Alternator over charging at high RPM. Any ideas?