How do you fix an audio cassette tape that plays through most of the record?

How do you fix an audio cassette tape that plays through most of the tape but slows down or stops completely through certain area of the tape. This has happened with two tapes so far. Maybe for 5 to 10 minutes or less about two thirds through the tape on one of them and towards the end of the other. Both tapes are older AMPEX 90 minutes tapes. I am trying to record some 20+ year old live recordings of one of my bands onto my computer hard drive and then preserve them on cd's or dvds. I have cleaned the heads etc of the tape player. These tapes haven't been played for years. I've been saving them for this purpose. The player is an AIWA HXPRO with low hours that I bought from a studio for this particular purpose.
I have taken cassettes apart and replaced tension springs before but I don't want to mess this up and wasn't sure if someone could tell me the best way to proceed would be.
Thanks for your help!