I have a 67 C-6 in my 65 country squire and was having a long delay in the 2nd to 3rd shift but a modulator valve and hose fixed that problem but it still has the wimpy shift it always had,at low speeds it shifts into 3rd nice and firm but when the speed is up and on the gas it wants to slide into 3rd. Its been doing it since I installed the motor and trans 4 years ago and has not gotten any worse and the fluid is still red but I would like to know what would cause it. Jeff