Rev Counter

My Audi 90 V8 1994 has the following issue. While driving along all of a sudden the rev counter drops to zero, after 2-3 minutes either the oil light, seatbelt light, or temp light start to glow. First faintly then getting brighter and brighter. Sometimes the rev counter starts again and the lights go out. Never happen right after starting, normally after 15 minutes of driving