Strange vibrations, maybe?

[spyder803]

I have a 2003S with 52k miles. I recently (week ago) changed my tires and wheels and have developed a vibration. The car tracks straight and I have no issues until 70 mph in 6th gear and then I can feel a pulsing type of vibration through the accelerator pedal and a very slight side to side movement in the steering wheel.

I am bringing it to have the balance checked but what I noticed last night is that if I plow past 75mph the vibration goes away. Also, if I stay in 5th gear I do not feel it.

To some degree it feels like a miss when I accelerate from 70 to 75 in 6th gear.

Any ideas? I'm going have the balance checked on the new tires this morning but have some concerns that this may not fix my issue. Any other items I should check or consider or do you believe this is just a balance issue with the new wheels and tires?

Thanks,