Could be carbon buildup-when the car's cold, any carbon in a cylinder, as I understand it, acts like a sponge and absorbs some of the fuel. This changes the mixture to lean, and therefore causes the car to stumble.
I had this problem with my M119 V-8, and a couple of cans of BG 44K took care of it. Good luck!
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car