My engine is having problems getting up to speed

My Renault Megae 1.6 Dynamique VVti is shaking really badly when I drive it, and it does not pick up speed even when the accelerator pedal is pressed to the floor. It really struggles to pick up speed and just decelerates. At times it begins to judder really badly like when a car is about to stall, but my car doesn't stall just shakes like that repeatedly. At times I have parked up, switched the engine off and then restarted the car, it allows me to drive off fine but then the problem within seconds returns. The message displayed on my dashboard is 'check injection' with the engine symbol flashing alongside. What is wrong? I've taken it to so many garages and I have had a mechanic replace one of the four connected to the engine, the problem subsided but didnt go away! What could be the cause? It has been recommended to me to get a thorough diagnostics done to determine if there is something wrong deeper within the car like with the brain box for example. Is this necessary, because this is all getting expensive. Going by the symptoms described, can you enlighten me with the possible areas of concern so I have some direction of investigation of the most probable cause for the problem. The mechanic who just reviewed the car, checked it on his diagnostics computer and the error message was the ijection, he was unable to determine which one. However last December when I had this problem, I took it to a different mechanic who identified which injector needed replacing as he pulled one out, one by one, whilst the car was running and when the car conked out when he disconnected one of them, he said that this was the injection which was causing a problem and replaced that, total bill £100 inc labour and part (second hand part). He said that this problem should be avoided now for the next 12 months as he solved the problem, I will take the car back to him now but I'm just not confident in any mechanic solving the problem.