Vauxhall Meriva starting car

i only got this car and warranty ran out had it in with the ones i bought it of several times. they dont have a a problem with the car. when i start the car it doesnt start it goes dead striaght away it only works when i accelerate a bit then works perfect does not cut out when driving at all no noise comes from it sounds great just that i have to accelerate to start it nearly all the time does anyone know what could be causing it. its starting to do my head in and people i bring it to think im nuts.its like it hasnt enough power to start it, but once u do that it works driving until u stop the car and have to do it all over again.