You could have loss of vacuum. You might check the vac reservoir and your lines. I check vac lines with starting fluid. With the engine running spray it on the hoses, if there is a leak you will hear a difference in the engine. How is your speedometer working? It may have something to do with a speed sensor also. It could have an efect. I once had mine go bad, the speedo still worked but the trans would not shift out of first. Sounded strange to me but it could be bad and the cruise does work from that.Let me know what you find.