This one has me stuck.

I have an algorithm for creating a tube of n sides in which the vertices are ordered as a triangle strip [TUBE ALGO].

The problem is if I have two tubes and I want to connect them to make a larger tube I am currently using the following indices:

            debug_Tube_Joined_Indeces[0] = 0;
            debug_Tube_Joined_Indeces[1] = 1;

            debug_Tube_Joined_Indeces[2] = 2;
            debug_Tube_Joined_Indeces[3] = 3;

            debug_Tube_Joined_Indeces[4] = 4;
            debug_Tube_Joined_Indeces[5] = 5;

            debug_Tube_Joined_Indeces[6] = 6;
            debug_Tube_Joined_Indeces[7] = 7;

            debug_Tube_Joined_Indeces[8] = 8;
            debug_Tube_Joined_Indeces[9] = 9;

            // =================================================

            debug_Tube_Joined_Indeces[10] = 18;  
            debug_Tube_Joined_Indeces[11] = 9;

            debug_Tube_Joined_Indeces[12] = 16;
            debug_Tube_Joined_Indeces[13] = 7;

            debug_Tube_Joined_Indeces[14] = 14;
            debug_Tube_Joined_Indeces[15] = 5;

            debug_Tube_Joined_Indeces[16] = 12;
            debug_Tube_Joined_Indeces[17] = 3;

            debug_Tube_Joined_Indeces[18] = 10;
            debug_Tube_Joined_Indeces[19] = 1;

            // =================================================

            debug_Tube_Joined_Indeces[20] = 10;
            debug_Tube_Joined_Indeces[21] = 11;

            debug_Tube_Joined_Indeces[22] = 12;
            debug_Tube_Joined_Indeces[23] = 13;

            debug_Tube_Joined_Indeces[24] = 14;
            debug_Tube_Joined_Indeces[25] = 15;

            debug_Tube_Joined_Indeces[26] = 16;
            debug_Tube_Joined_Indeces[27] = 17;

            debug_Tube_Joined_Indeces[28] = 18;
            debug_Tube_Joined_Indeces[29] = 19;

Where 0 - 9 belong to the first tube and 10 - 19 belong to the second tube.

When either the first or second tube is rotated slightly, extra triangles are created that shouldn't be there.  

Where have I gone wrong?

Thanks.

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[TUBE ALGO]
 
            /*
             * Tube vertices are ordered as follows (as a TriangleStrip):
             *  
             *  [Top]
             *  1   3   5
             *  |   |   |
             *  |   |   |
             *  0   2   4 etc.
             *  [Bottom]
             *  
             */
            for (int i = 0; i < Sides + 1; i++)
            {
                double theta = (2 * Math.PI * i) / Sides;
 
                // Create and store the vertices in local space coordinates (Use World_Transform when drawing)
                Vector3 vertex_Position = new Vector3((float)Math.Sin(theta), -1.0f, (float)Math.Cos(theta));
 
                // [Bottom] polygon end of tube
                Vertices[2 * i + 0] = new VertexPositionColorTexture(vertex_Position, Colour, new Vector2((float)i / Sides, 1.0f));
 
                // [Top] polygon end of tube
                vertex_Position.Y = 1.0f;
 
                Vertices[2 * i + 1] = new VertexPositionColorTexture(vertex_Position, Colour, new Vector2((float)i / Sides, (float)0.0f));
            }