A multiple misfire can certainly cause the engine to surge. You said you know what this was from - please elaborate.
The tachometer is just showing what the engine is already doing; it does not affect how the engine runs.
I still say there is something else going on in your computer system. Checking trouble codes is only one thing to check. Most scan tools can also check sensor values while the engine is running, and if you know what the sensor should be reading, a bad one can be spotted easily.
BILL