It is possible that your getting some flash back on your torch handle and you need to have a qualified person look at this and make repairs to it.
Perhaps something got lodged into the torch while it was stored or not used. I sometimes get bees making nests in the tips of my torch. My soldering area is outside and I am prey to these resourceful insects. I have made caps for each of my tips to help elliminate this bother.
Don't use your torch until you have someone qualified look at it. Check with the retailer that fills your tanks for a reputable repairman. As your supplier about flash back check valves for your torch. I believe it is important to put safety first in whatever you do so you can continue doing it.
Fred is right, with one other consideration. If the acetylene has also been sitting for awhile, it can degrade and would possibly burn hot. If a gas tank is stored, the lower the pressure, the greater chance of degrading or even contamination (depending on the gas).