I have seen tachs go both ways, but usually it wont act crazy,itll just kill of the spark at the coil.
Testing a pickup coil, it should be between 400 and 1300 ohms, and also, hook a DVOM to the two leads of the pickup coil, put it on A C volts , have someone crank the engine, and see if it makes about .5 volts AC
The only thing is, intermittents are difficult to test as when they happen , by the time you get setup on it, it may not provide conclusive results as the condition may not be present by then.
Check the pickup coil, then run the truck, and do the tach disconnect test.
Fact is stranger than Fiction