Coolant bubbling and steaming out of reservoir 99 Ford e250

Having recurring problem with coolant bubbling and spewing coolant out of
reservoir in my Ford E250. It does it when van is at normal temp and seems
to only run hot when almost out of fluid. I read on here it could be several
things from blown head gasket causing air in system, bad radiator cap (but
this van doesn't have one, only a cap on res), or bad thermostat. Van runs
rough when first started in morning. Don't know if that has any relation but
timing matches up. Is there any thing else I could do to determine what it is
before taking it to a mechanic? TIA