After driving around 8 miles on the way home i decied to open her up. Got up to 120mph in 5th gear and the power starter to drop? Am i right in thinking the car wasnt up to running temp? And was going into limp mode?

 

If the car have not fully warmed up, the ECU won't let your car open up to prevent damages. If that's not the case then... beats me Someone else will chime in and shed some light on this issue.

 

8 miles should be more than enough to warm the car up.

What do you mean by power *starter* to drop ? are you trying to say your car wont go faster than 120 mph ? if its AT, that sound about right, if its MT, you might want to check plugs/coils/cat

120 miles per hour ? You should slow down before u kill someone. or yourself.