Car problems....?

Hi i have a 1998 Isuzu Rodeo LS my problem is that once i reach about 60-65 mph it doesnt want to go any faster after that...it accelarates fine from the start but when im on a highway and reach about 65 it wouldnt go any faster...any suggestions?...i was thinking it would be my spark plugs if thats the case then i was thinkin of throwing on some bosch platinum series spark plugs...before i bought the car my uncle who used to drive it prior to me said it was fine on highways and hes a big dude too about 250 lbs...maybe i need to put a higher octane of gas maybe 91 although it only requires an 87 octane...plz any advice would be helpful so next time i go on a trip with my friends i can actually catch up to them...and btw even if i have a full car with ppl or im by myself it still slow...my oil is fine i put in a synthtic blend and my tires are on 35 psi.