Renault Scenic 1.6 Does not run smmothly in low gears.?

My Renault Scenic does not run smoothly in low gears.Have tried plugs leads and boxes. Has been scanned twice and no fault shows.Engine not noisy or sluggish.We can trace the start of the fault down to when the timing belt was changed but this has been checked and is perfect. Two different garages now do not know what the problem is. Car pulls off fine and then could start chugging until you take foot off the accelorator then you can gradually spped up and it is usually ok. Only happens in 1st or 2nd gears. Any advice please.