i would only like stage 6 if i had to money to get all the top bits like you have had done. but i havent the money so atm its just a budget build
ive sorted my stalker out ive taken the stage 6 off and put my gianelli pipe equivilent to a sito plus and it runs like it did before
i had one, it did the same, it migh just be the air filter, mine was... also could be the belt slipping....