Hard to start
 

IF the HG was taken care of properly i would assume, gauge or radiator failure. possibly a member with more knowledge on overheating problems could offer help???
hint hint, nudge nudge, bump bump...

Bah, hint taken...


No error codes stored in the ECU?

Definitely sounds like heat soak, but not in the traditional sense. It sounds like you have a component which is failing as the engine bay warms up. The first thing to do is to test the coils, fuel pump resister and relay, injector resistor pack and AFM RIGHT AWAY once the engine dies or starts messing up... You won't have a lot of time to work on it so try to be quick, but thorough. Once you reduce the number of possibilities you can use a strong hair dryer or heat gun to bring the parts up to temp to continue testing until one fails. The last time I had this happen the AFM was the problem (no error codes were stored though).

I forgot to mention that the car runs great, the only thing is that after shut the engine off it's hard to get it started...but when it starts it runs smoothly.
Is heat soak inevitable?? I never heard about this problem before, in other cars/brands...what to do in order to prevent it?

Thanks for the help!

I just told you what it sounds like and what to check first. I have seen this before on vehicles I've worked on; While it isn't common it's not rare either.