USB connection RAMPS 1.4

I have a MendelMax 1.5 with a RAMPS 1.4 and need some help. My problem is that I have trouble connecting a computer to the RAMPS when it's powered with 12V.
What I have to do to run the printer is
- connect the laptop, leave the printer without power
- open repetier host
- hit 'connect' in repetier host. The ramps connects, powered by the USB;
- then power the ramps with the 12V (by plugging in the 110V for the psu)
- in 8 out of 10 tries I can print; in 2 it looses USB connection the moment it's powered with 12V. Sometimes it stops in mid-print.
This works only with my trusty netbook running Win7. With an older desktop XP machine or with my newer Win8 laptop the success rate is much lower
All computers connect nicely to the Mega (alone), so it's not a driver issue.
I assume some ground problem, but can't really find a problem.
Any ideas? Please do not suggest using the SD card reader; tried that and it just opened another can of worms: out of 7 SD cards I could find only one works, sometimes. Maybe the same problem, don't know.

Any help appreciated!
edit:
I just re-read my own post and I realized it could be interpreted wrong:
The procedure I describe is the only way it will work at all; if I plug everything in and just fire it all up the computer won't connect to the RAMPS.

Edited 1 time(s). Last edit at 03/03/2015 08:57PM by crashsalot.

When you do all of that, is the laptop connected to its power brick , or running from its battery?

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

How is your 12v supply connected to 5A and 11A busses?

See:
[reprap.org]

and check your wiring.

USB reset may be occurring, but your laptop should be running on battery and filter
out power spike from turning supply on, but upset could be coming from arduino usb fidi.

Any donk donk (US connect / disconnect sound heard?

Try connecting it up via powered usb hub.

Quote
dc42
When you do all of that, is the laptop connected to its power brick , or running from its battery?

I tried both ways, but it doesn't really change anything.

Quote
cozmicray
How is your 12v supply connected to 5A and 11A busses?

See:
[reprap.org]

and check your wiring.

USB reset may be occurring, but your laptop should be running on battery and filter
out power spike from turning supply on, but upset could be coming from arduino usb fidi.

Any donk donk (US connect / disconnect sound heard?

Try connecting it up via powered usb hub.

I'll check the wiring again when I'm at home. Never thought about that, but I do not hear the Windows disconnect sound.

I noticed something else: when the printer is powered up and I connect the USB cable to the laptop it changes the speed of the fan in the PSU. But that is the case with both laptops & the desktop.

Edited 1 time(s). Last edit at 03/03/2015 07:40PM by crashsalot.

I just re-read my own post and I realized it could be interpreted wrong:
The procedure I describe is the only way it will work at all; if I plug everything in and just fire it all up the computer won't connect to the RAMPS.