loose belt or do i need a new tensener???

[mat82284]

i have a 92 5.0 with a paxton sn89 unit on my car the car runs good and has no pings at all but at 4200-5000rpms my car jerks like it looses its belt sometimes it lets me hit 5000 but sometimes only 4200 when driveing then my car jerks and stops trying. when i turn my car on my belt squeeks and after 1 min it goes away i bought the belt stuff but didnt work and when i looked at my belt i noticed the tensener moves up when the car starts and when i rev it up hight the tensener moves up even more which causes a loose belt. i think my belt may be too long which causes the tensener to get pushed up with all the power?? or should i just buy a new tensener? i think its the belt because its a little too long. i have about 5 inch extra belt i think the extra belt causes it to move up and down?