TAZ 5 with unusual problem. Need help

I have a TAZ 5 with an E3D Titan Aero PRO tool head. A few days a go I had an issue with thermal runaway, but resolved it with a replacement thermistor. Now there is another problem. When I print a part, the first 30 minutes have no problem. However, after around half an hour, the filament starts slowing down then stops coming out of the nozzle. Ive tried multiple filaments, different temperatures, and adjusted the blower fan, but nothing works. The extruder motor continues to rotate, so it is not that. I even have a second backup Aero too head, but did the same thing. What could be causing this?

Does anyone have an idea how to solve this? I am getting frustrated with this

What filament type?

Try increasing extrusion temp by a few degrees and maybe a half turn on the idler. The tightening of the idler will help the extruder push filament through the hotend. Increasing the extrusion temp should ensure the hotend is keeping up with the filament extrusion speed.

If you are printing PLA at low extrusion rates, you could be having some heat creep induced jams. Reducing retraction to the minimum necessary, increasing layer thickness or print speed to increase flow rate, cooling the room and adding larger hotend cooling fans are all things that can reduce the heat creep problem.

Also, installing the heat brake into the heatsink while the heatsink is hot can help increase contact between the two parts, improving heat dissipation.

I am using PLA filament. I usually print around 210, but am trying it at higher temperatures around 220. I have a .8mm nozzle printing at 40 mm/second and at a layer height of .2mm. I set the blower fan to 50%. Does anything seem wrong?

PLA loves cooling! Its extremely hard to have too much. For a start, turn that fan up to 100% after the first layer or two.

when you took it apart to replace the thermistor, any chance you inadvertently reversed the fan? the fan bracket should be towards the heat sink.