My engine light blinks when going up steep hills in third gear for long periods of time. Any suggestion what it might be?

 

It's a misfire.

 

I know... But what could cause it to only do it up hills

 

Engine load and tired ignition components. You could also have other engine issues or a clogged cat. The issue would have to be properly troubleshooted.

 

What kind of cat would be recommended to replace it with or what should I do if this is the case?

 

Your options would be an OEM cat, aftermarket cat, midpipe with cat delete

 

Get the front of the car up on blocks and disconnect the midpipe from the manifold.

Shine a flashlight into the midpipe. It'll either be clogged or not.

If clogged gut it.

Check coils and plugs. Replace if necessary.

Go find that hill and try again.

 

Misfires kill cats, bad cats kill engines.
There is a sticky dedicated to misfires, anything you need to know is in it.

 

This is the thread that will help you solve your misfire problem. It really ought to be stickied in the newb forum as well as the troubleshooting forum.

https://www.rx8club.com/series-i-tec...t-here-222280/