Replacing the Transmission Fluid

I have 99 Isuzu Trooper. Lately, but not consistently, as I will be driving it, say 45mph, suddenly the RPMs will rear up, and I’ll have to slow down and put it in 2nd or 3rd (it is an automatic by the way) and then eventually it will shift back up into gear. I believe there is either something getting stuck in the valve and/or dirty transmission fluid with perhaps little pieces of metal. I would think the easy and best thing would be to change that out, but the A+ Transmission Specialist told me there may be burnt oil, and putting CLEAN fluid in there could actually be more damaging and he then suggested he take the entire transmission out and find out what part is getting burnt up, which of course costs me more that just a simple fluid change.

So, my question is: is it more damaging to put the clean fluid in, or is this guy trying to get an easy thousand from my pocket? What’s the best way to fix this problem? I would appreciate any suggestions, thank you!

Get away from the transmission chain shop before they empty your wallet.

There is NO downside to changing transmission fluid, and anyone who tells you otherwise cannot be trusted.

If the transmission is already damaged, it’s damaged, but it won’t be damaged by putting in clean fluid.

Find an independent mechanic, NOT A CHAIN SHOP, and have the transmission fluid changed.

Then cross your fingers.

I would definitely get a scanner on it, check for codes first but there are things that would be causing the downshifting like you are feeling. A TPS is a perfect example. They wont always turn on the CEL but they could be the cause. You need to find out whether the downshifting is computer commanded or not.