Maybe it's your timing, and not the pressure at all.
Keep in mind that the shift key needs to be held during the typing of the following character.
The older mechanical action on a typewriter would give you instant feedback that the key was not held down completely. A computer keyboard, even one with mechanical switches, would not provide that same physical/optical feedback. And, mentioning keyswitches - have you ever tried one of the better mechanical computer keyboards? Look for one with cherry switches.
The old Apple Extended keyboard is well remembered for being one that 'types' very well, with a very good touch.
Maybe a few minutes a day with a typing program would help to 'retrain' your shift finger.
Here's one example of a typing app - http://www.versiontracker.com/dyn/moreinfo/macosx/24438
Serendipity is a lucky guess !