set timing? checked to see the vac advance in distrib is working and connected properly?

Until computer controlled engine timing* came along to forklifts, most forklift mechanics "in the know" considered "dual fuel" to be a cruel joke on forklift mechanics, since we knew that in order to get a conversion from LPG to Gasoline (or the other way around) to run correctly required a change in the ignition timing by about 5 degrees, and we usually "split the difference" with a slight advantage to whatever fuel system was most likely to be used.
if your truck is "pre-emissions controls" be sure that you run all the fuel out of 1 system before you try the other system, or you may have too much fuel in the combustion chamber, which can sometimes cause symptoms somewhat like you describe.
*Computer controlled emission dual fuel trucks have 2 sets of computerized settings, 1 for LPG and 1 for gasoline, so the dual fuel actually works correctly, but the computer learning process requires 2 sets of "learn", 1 for each fuel source.