Corsa b 1.2 will lose power when going uphill and check engine light comes on

My vehicle is having an issue there are two things happening, but I get the feeling its the same thing causing both problems. First is when starting the car it starts the first time every time with no problems and idles fine but sometimes (not every time) when I pull away it will pull away and then the car will just stop moving. If I put the clutch down and try pulling away again it pulls away to about 5-10 mph and wont go anywhere unless I repeat, but after doing this a few times she (wakes up) and is suddenly fine. The second problem is when driving uphill at speed (it does not seem to happen when going slowly in a low gear) only if I am in for example 3rd gear going 40 mph I can feel the engine losing power and acceleration then the check engine light will illuminate the car will continue up the hill but will slowly (depending on the length and steepness of the hill) decelerate on some longer hills I have started in forth gear doing 60 mph and by the top I have had to change down to second and an now going 20 mph with my foot planted all the way up. Then at the top of the hill once it goes flat(er) the car will go back to normal and start accelerating okay and then the light will turn off and will have no issues until the next uphill. I hope you can think of what is causing this issue it maybe worth noting as well when going up the hill with engine light on if I release from the accelerator when its about half way the car seems to gain a little speed for a second, but only at a specific point when releasing the throttle it does not work when pressing it. Many thanks.

Dan