Hi Chuck and Chris,
The external clock input on the 6802 is divided internally by four to create an internal two-phase clock (one phase is output as signal "E"). So, with a 5MHz input, the cycle time is actually 800ns, which well within range of the Intel D27128 with 250ns access time I mentioned before.
There is also the NOP exerciser which can be enabled with jumper P503. This is good for getting to the bottom of possible processor and/or bus buffer issues by looking at E, address lines, and other signals as the processor is looping. Its operation is described in the service manual.
I do recall one instance of a bad processor, but granted, they are rare.