Hows the Viscous Fan???
If the temp drops with the additional cooling effect of the Heater Matrix, I would summise that the rad/fan assembly isn't cooling efficently....
Do the news paper test....
Get the engine up to temp, then with a tightly rolled newspaper try and stop the fan, if the fan slows or worse stops, the VC unit is knackered, the papper should get shredded on a good unit....
Also make sure you have bleed all the air out, because the LPG vapouriser is mounted high in the engine bay, it can make getting all the air out difficult....make sure you burp the hoses to try and get all the air out.