Wrx.wrx

Hi I am new to the Subaru scene and I'm in need of some help please. I own a wrx bug eye impreza I've owned it for bout 4 months and the 1st thing that happened that was under steady acceleration my car started to 'stutter' especially when holding a constant speed of 60-70mph when that happens the engine management flashes at me so i slow down to 50mph and its fine and light goes off, but about 15-20 mins later of driving the engine/emissions management light comes on permanently! I have send to try and get it diagnosed but they can't find a fault and just reset it but as you would expect the same things happens any ideas what the problem might be?

The 2nd problem is that when my car warms up it just goes strate to the H only when I stop at lights then pull off to bout 40mph does the temperature fluctuate to about normal but then nearly immediately it's back in the H. I first noticed this problem after a long trip on the motorway might be a coincidence just thought I'd mention it! Now my fans are working, there is the correct level off water/coolant and oil is correct. Once again guys any ideas why?

My car has got cat back exhaust and induction panel filter and has dump valve that's it no other modifications

Please help or can any of you recommend any where to take it in the stoke on Trent area?

Cheers