Sure sounds like it is slipping. If the car has been doing that since you got it, it could be that the kickdown linkage has always been out of adjustment and needs adjusting. I would start with that. You could be putting "just" enough pressure into it to make it seem kinda normal. At WOT make sure the lever on the trans is all the way back. If that seems to improve it some, next I would call a reputable torque converter company and give them your cam specs and see what they recommend for a torque converter. 3000 seems high to me. If your running a mild cam I would think a stock stall or maybe 2200-2400 would be sufficient. There are a multitude of factors, car weight, cam (power band), rear gears, and of course usage. IF it is going to be mostly a street car then in my opinion 3000 is to high. In 1973 Chrysler pushed the limits, much to everyone's suprise, when they put a higher stall converter in production vehicles which was still less than 3000. A given torque converter may stall at 2,500 rpm behind a small block with 300 lb-ft of torque, but provide 3,000 rpm before reaching stall speed behind a 440 which belts out 450 lb-ft of torque at stall rpm. It varies.