If you replaced the fueltank and the fuel lines this is the order I would work in:
-Retune the engine, go back to factory settings and retune it for optimal performance. To me it sounds like it is running too lean since it happens after around 3 minutes.
When you start an engine it will heat up for around that time, after that temperature will stabilize.
-If retuning doesn't solve it try a different carburator or replace all the seals in the carburator. I experienced a leakage by a damaged O-ring on the LSN after adjusting it a lot during break-in.
-If the problem still persists try a different exhaust. It could be clogged or damaged. This year I had a brand new exhaust which wasn't manufactured right. When it became hot one of the inner chambers could move around and messed up tuning a lot. Took me a while to figure that one out.
-Last step would be sealing the backplate and the entry for the carburator. Personally I never had to do this to an engine. I believe if this is needed one of the parts is seriously out of spec and should be replaced.
If you keep having problems try making a video of running the car so we can see and hear the engine tune.