Speedometer Failure

When driving home at about 60 mph last night in my 96 Lsi ,I experienced a sudden oscillation of the speedo needle swinging between 20 and 45 mph , even though my road speed hadn't changed .It lasted for about two minutes and then the pointer dropped to zero and wouldn't budge again . I checked the cruise control which then would not operate . About 5 minutes later the "check engine " light came on .
I'll take it to the dealer tomorrow and report back later . I'm not sure if the speedo is cable driven or just an electronic pickup on the SVX .

Alan.