Wow! First time I've heard of this. I know many people here, including myself, have swapped out speedo only clusters for the speedo/tach clusters and have had no problems. Hope someone chimes in to help you; I can't imagine why it's doing this.
2003 ZX3 Zetec Turbo
1978 Fairmont Wagon 289/5-speed
1988 Saleen Conv. #368
2000 BMW M5