Re: steering shake/ tank slap
The higher your tire pressure the smaller your contact patch is. And the tires flexibility will be reduced to a noticeable degree. Which means you lose grip.
How many threads have we seen here where guys inflate to 42 psi and wonder why they lowsided.
And the following advice is always to reduce pressure to around 28 - 34 Rear and 30 -36 Front psi. Depending on the type of tire and riding style - commuting vs. canyon/mountain roads.
I would NEVER inflate to anything above 32 psi for the rear, unless riding two-up. even then, I'd max at maybe ~36.
There are tons of threads discussing tire pressures, and believe me - 42 psi is ridiculous.
18 psi on the front seems way to low for a normal sport/street tire as well. 30 -34 is more like it.
I've been experimenting with tire pressures on my bike for the last three years. I record the ambient temperature, and cold psi before a ride. Then I pull over during spirited riding, check tire pressures and heat (with a laser temp reader) in the tire. The Dunlops, Bridgestone, and Michelins that I've run: they are all pretty close and I run Cold pressures of about 28 Rear and 30 Front. (That's when cold temperatures are in the 60 to 80F degree range.) That's where the contact patch, flexibility, and stability is once the tire is warmed up.
I know this doesn't exactly address the headshake your experiencing, but could affect it somewhat.