Rostock problems on the outer regions.

Hi !

I have built a RepRap Rostock Delta 3D Printer, with ramps 1.4 and modified marlin fw.
Small prints, 100-120mm wide is no problem, almost perfect.
But when i print bigger things, thats when the problem starts.
If i keep the temperature at about 185-190 degrees celsius its ok in the middle, but along the sides, it's almost like the hot end is getting colder, the filament won't go through the nozzle and the extruder-motor just keeps jumping back cause it can't hold the force...
But when it goes back to the middle its fine again.
i tried increasing the temperature and when i get up to about 230 degrees, it works.
But then it like its flooded with filament and there is just too much filament coming out, also the whole thing collapses in the middle, probably cause the high temperature.

It looks like its moving slower on the outer parts, but maby its suppose to do that cause of the delta...

Have i missed a setting for this or is there something wrong in the fw, i think there is a newer version but i thought i ask here first if someone else got the same problem.


With regards
Mats
Sweden