>The bike runs good when the temperature is 50* out, but really runs bad (the >cutting out thing as before) at 75 -up.
If this were a mixture problem, this sounds like too rich, try turning off the gas and see if you speed up just before it runs out of gas.
>top groove works best
If this means the top groove in the needle, which lowers the needle the maximum amount, this also points to a too rich problem. Have you tried running with no air cleaner?
Another possibility is that you have a heat related problem in the ignition system.