Speedo falls to 0 when I use turn signal...

[lilbobyleffew]

I just purchased a 91 350sd and the car is in immaculate shape but there is one ailing problem that has me worried. The speedometer will be working fine and when I use my turn signal it will just go limp and fall to 0. It does not do this all the time, but most of the time when it happens I can get it going again by simply pushing in on the trip/dimmer knob. Anyone ever experience anything like this and what do I need to do or check? Thanks in advance for any help.