Bed loosing temperature

Hi. I have a 3D printer without enclosure equiped with an MK3 Alu Heat Bed. My preheat values for ABS are 230/90 and when I preheat the 3D printer there's no problem keeping the temperature fixed. But when I start printing my bed temp goes down til 85 sometimes and then rise up slowly again till 90. And this happend for about the first 7 minutes on the print. I'm using an Mean Well 320 12V 25A with 2mm thick cables soldered straight on the ramps board. Is it normal this to happen? And more important, if i try to rise up the temp during the print from 90 to let's say 100 it will not increase it. After three minute or so I have the message Printed Halted on my LCD and I must to reset the 3D printer. It's like the 90 degree temp is freezing. Offcourse, If I want to lower the temp from 90 to 85 it will decrease imediately.

I've had similar issues (I think I posted something here about it). Turned out for me to be wonky thermistor connection. I replaced the thermistor completely with all new wiring and connector. That took care of it. If you're using any new E3D hot end, the new cartridge thermistor is the shiznit.

What does it mean shiznit? )) So your advice is to use the same thermistor for my bed as e3d use for their hotend?