Fluctuating temperatures while driving

Okay, I've got a '67 Dart with a stock 318 lower end and heads, edelbrock intake, edelbrock 4-barrell, and dual exhaust.
Recently my car started running hot, on the highway or in town. Sometimes it runs at perfect temperature (from my stock gauges), but then other times it will heat up to the hot end of the gauge, or even peg it out.
Speeding up usually drops the temperature some, but then again sometimes slowing down will cause temp to turn as well.
I have replaced the thermostat (195 F), installed it the right way (spring down into the manifold), sending unit, and replaced the stock 4 blade fan with a new 6 bladed flex fan of the same size (installed the right way, evidenced by much more air flow into the engine compartment!). I even flushed out all of the old coolant and replaced it with new.
The frustrating part is that sometimes the temperature is perfect. Then out of the blue, it starts heating up quick.
Any ideas?