2005 caravan rostra cc problem

I purchased and had installed a Rostra micro-cruise 4 for my 2005 Dodge Caravan (4 cylinder engine, automatic, power nothing).  While it took forever to find the tach signal we finally got everying hooked up and working - at least we thought we did. 

When we engage the cruise, it holds speed for a second or two and then dis-engages.  Any ideas or suggestions as to where the problem may be would be appreciated.

Thank you,

First I never use the tach connection. It always seams to cause problems. So I tyrically ground the blue wire.

Now I have never had a problem putting a cruise on a Caravan, but I also don't use the micro cruise 4. Can the MicroCruise 4 be set to 36,800 pulses per mile (thats with the 250-4369 pulse adapter)? Or do they make a pulse adapter that will allow the microcruise to read the Caravans VSS signal?