Kia loses power

I’m working on a 2002 Kia Rio manual tran. Just recently it started losing power. When I first start it up I can go 2-3 minutes just fine, but then the engine seems to just bog down. The rpms are not going up (so hopefully not a transmission). I struggle to get to 10mph. I checked and there are no codes evident . Or rather there were P0301 and P0304 misfire codes, but I cleared them out and drove it around a while and there are no error codes now. I’m at a loss because of having no codes to work from.

I checked and there was a small crack in the exhaust manifold but it did not go all the way through and I repaired it with Quik Steel and it made no difference. I’m not sure where to look next other than to start throwing parts at it. The vehicle is new to its owner – within the last 18 months – and I know it was maintained for this period of time but don’t know what was done prior.

Thanks for any advice.

I would check fuel pressure.