96 blazer-black soot and lack of power on steep inclines

I own a 1996 chevy blazer 4.3L V6 fuel injection, 115,000 miles. The SES light is always on. I noticed that when it is started and when you get on the gas, black soot is thrown on the ground and is in the tail pipe. Now just recently I drove it down the thruway, As soon as I got to 65MPH the SES light began to flash and it acted like it didnt want to go any faster then 65. The Temp was normal, The oil pressure was normal, The battery gauge was fine. The wierd thing is that once I dropped the speed to 62MPH the SES light would stop flashing. The same thing happened to me when driving up a steep incline on a mountain road. I had my foot damn near to the floor and it just barely crept up the hill, At about 1/2 way through climbing the mountain the SES light began flashing. Those are the only 2 times the SES light flashed. The Blazer runs great at normal speeds and on normal terrain. It starts great cold or hot. It does seem to have a jumpy idle in the cold but idles nice at stop lights and stop signs. Any info would be greatly appreciated...