I've had my 98 LS400 for around 4 months now, and from day one its suffered the low speed hesitation issue when executing low speed turns or any scenario where you slow right down, then speed up again without stopping. The revs build, but speed doesn't and then with a jolt, everything syncs up and you're launched down the road somewhat embarrassingly.
I've taken a 'lazy git' approach and reached for a fuel treatment, hoping to clean the injection system, prior to removing the throttle body for cleaning at a later date. I'm a sucker for wonder products, and this one seemed to have some decent reviews:
http://www.powerenhancer.co.uk/archoil-ar6200-fuel-modification-complex-high-concentrate.html
Over the past 3 months (circa 1k miles), i've used the highest recommended dose of 1ml treatment to 5 litres of fuel on 3 occasions now, and have noticed a difference straightaway in terms of improved pick-up, throttle response and acceleration.
Best of all, the hesitation is vastly improved - i won't say its gone completely, but in the last 50 miles or so, I can't say i've noticed the aforementioned symptoms. I've not checked mpg, but most reviewers seem to have experienced some improvements.
The treatment is not cheap but appears to work in terms of general performance benefits (throttle response, pickup etc) and protection against ethanol content.
I'll report back in few months time to see if its still working.