1976 Ford Thunderbird Idle Rough When it warms up

I have a Ford Thunderbird 76, Base model with the 460 engine in it. I just got the car tuned up routine maintenance, changed spark plugs, new leads, fuel filter, air filter, oil change oil filter. the car runs great until it warms up, after about 60mins of driving the engine idle becomes really rough and it loses power, the note on the engine changes and can sound hollow. It has never done this before the service, it has always run fine... I took it back and told them the problem and they adjusted the idle on the carb around 50RPM's. once again it runs great until it warms up... then it shakes and loses power again, it doesnt stall just runs like a dog. Any ideas??