Turnigy 9x+DYI Headtracker - radio hangs

[yay]

I got into troubles with my Turnigy9x and DYI HT (http://code.google.com/p/open-headtracker/) recently. I had bunch of flights during the spring, but then my radio started to reboot or hang. I thought it is some 9x issue but, noticed it only happens when HT is connected. When the radio freezes (screen goes blank, no PPM signal) disconnecting HT makes it work again.
There is a problem when I'm connecting HT on or off, and the radio is on - it sometimes hangs/reboots.

I didn't noticed any issues when connecting DX6i as a slave. Works all the time..

My setup is: turnigy 9x with er9x, Chainlink LRS and DYI HT. PPM OUT with 1k resistor mod. Software on both, 9x and HT is not up to date, but i didn't expect issues because of that.
I'm connecting HT no by jack port but a socket soldered to PPM IN (one of the jacks pins) and the radio battery +, GND directly. HT was powered with a 9x's 3s lipo, but changing to 6V (via bec) didn't changed anything.

Does anyone know what may cause radio to hang? May something like a diode or resistor into a PPM IN line solve the issue?

[jhsa]

what are the levels of the head tracker's PPM output?

[yay]

It should be 5V, since it's arduino(328p) digital output pin. I have only a multimeter - won't be usefull measuring PPM signal levels, right?

Edit: multimeter shows 4.16V

[Phelec]

I Have the same set up and the same problem. I have found that adding a 1k resistor in series with the ppm signal coming from the DIY HT has stopped the problem of hanging. I scoped 4.5Vpp on the HT side of the resistor and 5.4Vpp on the 9Xs side of the resistor.