when you say the car slows back down to 70, do you mean it literally fights you for control of the speed? and you physically cannot push it above that without fighting the car?
as for the starting issue, sounds like it might be more than just the 02 sensor, perhaps the lamba is one to consider.
I would start with the basics though, I wonder if you are starting to get a small amount of air in the system, sufficient enough to restrict the power of the car, but not enough to stop the engine. Replace the fuel filter, and consider the housing also, replace the seals as well and see where it goes from there.
Do you have a code reader? if so best check to see if there are any other codes stored within the car?