Sparkplugs die or get weak and often there are no external signs.. get a new plug of the proper type. But if the bike starts ok with the other plug, i doubt a plug is the problem. If the bike runs fine above 10 mph, i doubt the problem involves ignition.
Fuel mix ratio problems, on the otherhand, most often cause a bike to run crappy at some speed range.. only at idle, only at top speed, or at some range in between.
Besides the advice people have offered, old gaskets shrink and leak air.. vacuum leaks around the intake manifold upset mixture ratio. Lots of other things are possible causes of crappy low speed running.
More detailed info about that bike and exactly how it's acting might help people narrrow causes down to one or two possibilities.