I knew there was something I'd missed on this thread on the first read - it's not clear if you've had this bike since new, but if you have, then:

100 miles/wk over 4 years = 4 x 5,200 = c21,000 miles. This is quite a lot of milage, and will have put a fair amount of wear onto drivetrain components, rims etc.

I'd agree with the postings made above (though will stay out of the stem debate) but would also say take a look at the state of the chainset, cassette, chain, rims, bearings (hub, bottom bracket, headset). I'm assuming you've been carrying out routine maintenance, but at this mileage it might be worth looking at these 'high mileage replacement' items to see if any failures are imminent, or if ,e.g., chainrings are worn to the point where replacement would bring about a big rise in performance.

Just a thought.