Personally I found MRTG as the most useful tool to measure the load (since MIB are constantly updated and keep historical record). Just to wide a little bit more in your previous query ....
"Average number of bits and packets transmitted per second in the last 5 minutes. If the interface is not in promiscuous mode, it senses network traffic that it sends and receives (rather than all network traffic). The 5-minute input and output rates should be used only as an approximation of traffic per second during a given 5-minute period. These rates are exponentially weighted averages with a time constant of 5 minutes. A period of four time constants must pass before the average will be within 2 percent of the instantaneous rate of a uniform stream of traffic over that period.
Note The 5-minute period referenced in this output is a load interval that is configurable under the interface. The default value is
5 minutes. "
About the traffic shaping issue, if you configured traffic shaping becn makes sense that the queue will not hold packets unless you receive becn's .
what shows up in the show frame-relay pvc and show traffic-shape statistics when is under heavy load?