vt ss keeps jugging when i reach 30ks????

[Emma07]

hey,i got a vt ss that keeps pulling back when i get around the 30k mark,its starts to jolt back and forth until i get into 3rd gear,have changed spark plugs and used injector cleaner, it did start to go normal after that for a few days and has now buggered up again so i changed the catalytic converter (i found some of that beehive crap near exhaust)and put a new fuel filter in but nope still dont go properly.....any ideas????

 

[accentstencil]

A dodgey park plug lead would be a likely cause.