ok here is the question i got a 1991 camry sv21 3sfe 2lts 4 cyl its a manual

and somedays when i am flying up the highway and i get into the higher rev range (slap it down a gear)to go over take some people and when i getnear the redline it doesnt seem to have any more power it just sits therenear the redline not picking up anymore speed feeling sluggish.. but otherdays when i do the same thing it keeps picking up the speed
is this caused by anything bad?? plz wbs

 

could be the temperature outside.