Car Shakes When A/c Is On

[TAZ's240sx]

A few months ago i was driving on the freeway with my a/c on, I was going to 65-70 and everything was fine. Yesterday I was on the freeway again with the a/c going at the same speed, But i decided to go a little faster and thats when my car started shaking. Is this normal for a 89 240sx with 100k on it or is something wrong with it. And I was going to 85mph when the shaking started. while on the subject on a/c's, will my car go faster if I remove the a/c. Just wondering on that cuz a friend took his a/c off and said he feels more power know that he took the a/c off.

----------------------89 240SX INJEN INTAKE,ACT CLUTCH,AND A REBUILD transmission:)