You have a vacuum leak in the back of the cluster, probably the economy gauge or a line going to the climate control. Are all of these items working properly?? The fact that it is evident with closed throttle (high manifold vacuum) and goes away with open throttle (decreased manifold vacuum) is a dead giveaway for vacuum problems. Check all of the above mentioned systems. Light switch bulb replacement usually requires removal of the fascia panel and the switch. Not sure on your car however..
1987 BMW 325
[This message has been edited by jeffsr (edited 06-04-2000).]