Car is a 93 300D 2.5Turbo

Hello all. Ive noticed something funny today. Sometimes my tach will not work (register 0 all the time). When this happens, the car is sluggish (turbo is not kicking in). When the tach is working, the car drives fine. How can this be? Also, the cruise control DOES work when the tach doesnt.

I checked the Overload protection relay and both fuses are fine. Should I go ahead and replace the relay?

any info greatly appreciated.

__________________
93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K