Tach drops when warming up

[richsho]

Here is my problem when my 89 sho reaches a certain temp,tach will drop to zero still runs but 'misses' for about 3 mins.Then tach goes up and car runs great again,I think it is heat related sensor changed my cpk a month ago.once she warms up no more missing, always the tach drops to zero when temp gauge hits the begining of normal. any body have any idears