After serious thinking Adam Funk wrote :
I've been using a late relative's spare blowtorch that he left at my
house ~20 years ago. It consists of an orange tank with a valve, a
rubber hose maybe 2 m long, and a handle/outlet with a valve, & two
hose clamps. (I don't use it very often.)

When I used it last week, I had occasional problems with it: switching
between "neat blue flame" & "orange flamethrower", occasionally going
out, & leaving some wet-looking stuff on the nozzle. Does this just
mean the tank is almost empty, or is there something wrong with the
handle/nozzle?

Should I replace it with a newer model? Any recommendations for
occasional plumbing jobs?

Sounds like liquid gas (propane) getting through to the burner. Tank
needs to be upright, with the regulator at the highest point, to
prevent liquid coming out of the jet.