>>Cylinders 2 and 6 were causing the sputtering.<<
And you also said
>>works normally when accelerating and idling<<
If it was missfiring at idle, you would feel it and hear it.
It is not uncommon for some timing lights to act that way. Move the plug wires around and/or pick a different location for the pickup.
It sounds like you replaced the points not that long ago. The rubbing block under the point arm will wear more the first few hundred miles than it will in many thousands of miles after that. If you set the points on the close side and the block wears down, decreasing the gap, the dwell can increase above the correct setting.
At cruise the vacuum is the highest and the vacuum advance is pulling the breaker plate the farthest. You should check the breaker plate pivot pin for wear. The breaker plate on the 250 I6 is not the best design and dwell can change a little when the engine is reved and when vacuum advance is added.
Do you have a dwell meter?
My guess is that if you didn't notice the problem between last week and when the points were new (recent past?), then you need to reset the dwell and timing.
