left turn signal / cruise control problem

I drive a 1993 Chevrolet Silverado
When I set the cruise control for anything under 100 km/h and make a left turn using the turn signal, the speed needle jumps to the left
for a second then back in place. Like it shows I am doing 70 when I am doing 100. I don't lose power nor do I slow down. Only thing that happens is the needle moves. doing this without the cruise control engaged and the needle is not effected.
Any ideas on what might be causing this?
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