Moffet trailer problems

Moffet mounted trl and unit problem
Iv been driving this moffet mounted trl for a number of years now. But was given the new eveco hiways 2 weeks a go. My problem is when I am pulling the empty trailer it’s like driving on jelly. I have a steal plate on the head board. Iv tried moving my 5th to the front the back and the middle. Nothing seems to be working the trailer as fixed axles so I carnt lift 1st one up.any advice will be appreciated…

What configuration is the Iveco? if it’s a tag axle be VERY careful, if centre lift and you have a weight in the headboard it should be ok, but when empty it will still wander a lot, i pull a Knapen walking floor with a pig on the back, there are two ballast weights bolted in the front of the trailer, making just under 1.5 tonnes ballast. i pull it with a centre lift FH, although it does still wander when empty (really got to be wary of tramlines on the motorway!) it’s manageable, we previousley pulled these trailers with tag axle scanias, driving them empty was very scary!