89 3.0

89 with a 3.0. Was running but very hard to start. Check engine light was on and had a couple codes( knock sensor, and couple others. The strange thing is once it started it would run just fine. But if you let it sit it would be hard to start. The really strange issue started today, my heater wasn’t blowing very warm air. So I pulled the hoses and flushed the heater core. This cleared the problem with the heat, the truck started right up (idled a little high) it the check engine light went out. But when I would give it and pedal it would fall on its face and die. Then it seem to start die, start die etc. ow I cannot even get it to start. Prior to this the fuel pump, fuel pressure reg, filter have all been changed.
It also will just quick engaging the starter but I am guessing a relay is warm and not letting it complete the circuit until it cools down. I’m not really up on the computer trouble shooting but could manage with a little help. Any ideas. Like I said it was running just before flushing the heater core.