Shuts off when switched to low and smells when...

Question

Have a DYNA-GLO model RMC-50LPT propane wall mounted heater has a heat setting from 1 to 7 will run on high settings but when switching to settings 1 to 3 it will shut itself off . It running on a 200-250 lb outside stationed propane tank and its at 75% full.Also its sort of hard to get the piliot to light but i have always managed to get it to. When its running on eany other setting other than 1 threw 3 it has a smell to it sort of like when you shut a kerosene heater off and they develop that fowl smell?The heater is mounted in our garage,although the garage is very clean im wondering if the unit is dirty down in the orfice.Its not a brick infared heater it actually has the blue flames that raise when the setting is turned up . Thanks Shawn