What car is the install on?
You've said you have 2 of the same cars, one runs fine on LPG one doesn't, when you swap the LPG ECU from the one that runs OK to the one that doesn't if fixes the rpm detection issues on the one that doesn't?
What AEB2568 (King or Emer) is one that works OK (at least shows correct RPM reading all the time)? Is there a suffix after AEB2568 such as AEB2568 B/C/D on both of those ECUs (if so what suffix on both)?
Are 'extra injection filtering' settings setup the same in software on both ECUs?
What interface cable are you using?
On many occasions I have experienced the same type of RPM dropouts on AEB2568's that you are experiencing, which is why I asked the questions about ECU suffixes, extra injection filtering settings, interface cable and why I'm interested in your RPM detection settings. I found some unusual workarounds that we wouldn't expect to have any effect but do have an effect, I'll be better able to advise with the answers to those questions.
On the one hand the solenoid coil will pickup get heat conducted and radiated from the reducer and engine and it is electrically heated when its powered up, on the other hand the coil sits on the solenoid which is cooled by the flow of cooler liquid gas from the tank to the reducer. The coil doesn't usually get as hot as the reducer but this can depend on the install. Worth measuring Ohms across the coil when it is hot, or testing Amps the coil draws by temporarily wiring an Ammeter in series with it.
I don't remember ever buying bits from the website you linked to but I reckon a UK supplier like TinleyTech or LPGShop would sell you an ECU. Mick @Foxvehicles will certainly be able to advise you if TinleyTech will sell you one.