Renault Laguna limp mode

Hi when driving my laguna every now and then the power drops slightly especially in first gear. If i change into second gear the power returns and sometimes the heater plug light and fuel injection light both comes on together and when i stop the engine and start it up again both lights are out. Sometimes i don't get any problems at all and when the power drops its only for a split second but it never cuts out completely. I thought it might be one of the heater plugs not working but it starts up fine at mornings. Could anyone please help me and tell me why the power drops for a split second but like i said it picks it up when i change gear apart from everything else is fine. Thanks