EP3 04 acceleration and temperature problems need help big time!!

Hi guys,
Sorry if this is the wrong section and what not.
So basically I'm lost for what to do with my EP3 at the moment, it has a k20/k24a3 conversion done to it and on long weekend on the way back from goulburne it decided to stop running properly.

The engine is not blown and there were no fluid leaks, I was told it might be the Engine Coolant Temp sensor since it threw a "ECT sensor voltage high" error code in Hondata management and also throws "Multiplexer not present" all the time.

The temperature gauge on the cluster will show the engine temperature just below halfway when normal but if it runs for a trip longer than 30 mins and gets hotter it will simply cark it and drop below the Minimum temperature for the gauge. I replaced the ECT sensor today and it's still doing the same thing..

As we were driving home, we started losing power and the throttle got lumpier and lumpier until I couldn't accelerate AT ALL!! Not even 1km per hour, and the idle dropped to 500RPM and sounded like it was missing some cylinders.

I can drive it now for 20 mins perfectly normally, but then it will start to do it again when it gets hotter..

Any thoughts from anyone?

Oil and coolant is topped up, no milkyness in the coolant and fuses seem fine.

Really not sure what to do now as I'm nearly out of money to feed it and need the car for work :(

Thanks