Really bogging down in 3rd gear

Something to remember when troubleshooting a drivability problem, is that in most cases, a weak ignition will show up under periods of heavy LOAD. The engine does not have to be turning a lot of RPMS.

In the most basic way to explain it, when you have an engine under increased load, there is a greater demand on the ignition system. There's WAY more to the explanation, but essentially you have to have spark energy, spark duration( dwell, coil saturation) and ignition timing in parameters to avoid breakdown under load.

A fuel starvation, however, tends to show itself more at high rpms. The float bowl simply drains faster than it can be replenished. Lugging at low rpms will usually be trouble-free with fuel supply problems, where lugging with ignition issues will rattle your teeth out.