'98 Pontiac Grand Prix RPMs Issue

I really don’t think the MAF sensor causing your problems

The upstream oxygen sensor . . . you should have 2, pick whichever one is easier to get to . . . is in front of the converter. You might be able to see one of them when you have the hood up. Do yourself a favor and liberally spray it with wd40, or some other penetrant, and let it soak in for several minutes, before attempting to remove it

Cleaning the MAF sensor isn’t a bad idea as routine maintenance. May or may not be related to your issues. I’ve seen them cause general drive ability issues like stalling. Not out of the question that a dirty MAF would cause slightly harder shifts, but like Db, I’m a little skeptical that the MAF is the culprit.

The “sure enough” hard shifts, if you have them, may be due to the computer increasing line pressure to avoid further trans slippage. This has happened on my Buick since 50k miles or so. You’ll feel the 1-2 shift slip between gears, then the trans will adapt and each subsequent shift will be very abrupt with zero slippage. That is the computer’s way of protecting the trans, as slippage ruins the clutches quickly. It’s not “normal” in my case, but not the end of the world. However, I don’t have any loss of power like you’re reporting. In my case, I get a code (no cel), but I don’t remember what the code is. Basically, the computer detects a longer duration between shifts than there should be. Then it compensates by increasing line pressure and you get the hard shift like you’ve installed some drag racing style shift kit lol. But your issue may be different. No power loss for me, just some “slip/bang” action. Happens after the trans is up to temp in my case. Never happens when the trans is still cold (first thing driving in the AM).