71 mustang spark plugs

i have a 71 mustang with a 302 with a C4 tranny. i did a tune up about 5 months ago and i pulled 3 diffent plugs out of the motor and all 3 were white and looked new. from what i can tell the fuel mixture is right. it hasd plently of low end touque and power but as soon as it hits about 3,000 RPM it loses all of its power and then holds there for a little bit and then shifts. the shift is normal but you can tell the engine just kinda drops out. i thought before it might be because its a 2bbl carb but the plugs being white makes me think diffent. now i know the intake gasket has a small antifreeze leak but it runs onto the block and is very slow. i got 2 plugs from the passanger side and the other from the drivers. when i did a tune up i checked the timing and set it to what it said on the valve cover and then advaced it by a hair and i mean a like 2*. also if i hold the tranny in gear or have it in parm and rev the motor it dies off at about the same rpms and theres no water in the oil last oil change. any ideas? exhaust smells fine.