exaust issues

i bought an 83 kz 750 h4 in may of this yr as a toy. i didnt like the way it sounded so i cut the factory to the end of the mounts and pulled all the baffles out but the spark arrestors (im told thats what they were they were an inch in dia. and had holes drilled all around it) so i went to where the factory "trumpets" went in and put in 2 inch and a half turn downs of the same length. the problem i am having now is that i can redline the engine in every gear but top. i cant get over 75mph in top gear. this morning it died on me and when i recranked it, it was like nothing was wrong. it died again and on recrank it was the same way like nothing was wrong. does anyone have any ideas? i was told it was a back pressure issue and am thinking of running four straight pipes off the heads with a modified H pipe.