Renault Repair Questions? Ask a Mechanic for Answers ASAP
What engine do you have?
What year is the car?
Any management lights on when this occurs?
other than sparkplugs..anything else checked or replaced?
If its seems to run fine when warmed up..id say your best advice would be to find someone with a decent scanner with live data capability or scope to check Oxygen/Lambda sensor performance,heater side of it..sounds like to me a bad pre cat lambda sensor..but best to have them tested first ideally..I do hope this helps