RepRap temperature way off

Hi all, i hope there is a simpel fix for my problem.
i finally got all my axis and extruder calibreted and they are traveling the right distance (that only tog a few weeks, me being dyslexic and all). My problem now is, that the temperature is way off, it starts out at rum temp about 25-30C, then i set the hotend to go op to 180c, its starts heading, it thinks that it got it at 180c, men realy it's a 100c, when i set it at 230c, it peeks at 250c and tuns the header off, but it doesn't realy go higher then 100c.
any ideas whats wrong?

i uses a RAMPS 1.4 the Marlin 1.0.0 firmware
[www.youtube.com] <--it's at 450% speed but you can see that i'm talking about

What is telling you that it's really only at 100? Temp probe on your multimeter? Are you sure the thing that's telling you it's at 100 deg is accurate?

If the measuring device is accurate, I would check that the thermistor isn't shorted in some way and check all it's wiring.

okay, the multimeter prob, could be at a beder plads. i fixd that now.



set temp to 210 it shows 190 and is stabel,
set temp to 215 it shows 196 and is stabel,

And than it gets unstabel

Set temp to 220, multimeter jumps from 190 - 206 Pronterface shows 215 - 234
Set temp to 230, multimeter jumps from 190 - 210 Pronterface shows 216 - 254
Set temp to 235, multimeter jumps from 191 - 205 Pronterface shows 226 - 260
Set temp to 240, multimeter jumps from 190 - 213 Pronterface shows 226 - 270
than i peeks at 289-298 and shots down and i have to click "set" agen to keep it hot

what can i do/try at this point?

if i set the temp to 217c (178c - 180c) its fine and stable, if i set it to 218c on the order hand, it gos unstable and jumps around 215c - 231c ( 176c - 186c)