The never ending battle....

As you all know ive posted a lot threads on here wanting help . I intend to keep the car till its dying days , ive spent a fair amount of money correcting things , thing which are stressful and some which make me feel better, it seems that soon as I fix one thing another turns up lol. expected though I guess as she's 25 years old.

Changed the auto gearbox oil the other day, oil came out really black, however there where no metal deposits in the sump it was just oil. nothing stuck to the swarf magnet either. after the change it ran fine, but now its starting to take a little longer to change gear, almost like its reving really high to about 4000 + before changing, that is on a steady drive with no putting the foot down, understandably I live in a very hilly area, so ive tried this on flat roads the best I can, what could this be? TPS maybe?

Another thing is that since I removed my dash instrument cluster to check for corrosion on the plugs, my oil pressure gage from cold start is in between 0 and 4 before I done this the reading from cold was just below the 8 but it dropped down to between 0-4 after it was warm. I have changed the oil pressure sensor for a new one, what are the readings everyone else is getting? does this sound normal? can I buy an after market oil pressure sensor? and how easy is it to install.

THANKS