That old saying about "a poor workman blames his tools" does not always apply to guitar players. Put the arsenic away. :) Your axe needs the attention of a good tech.

A tremolo bar will cause problems for any Strat, but there are ways to minimize the effects. Even a hard-tail can have problems, if the strings are getting 'pinched' by the nut slots. A simple test for this condition is to press on a string between the nut and the tuner. The pitch should go sharp, and then come back into tune when you release the pressure. If it stays sharp, tension is being 'stored' between the nut and the tuner. This diference in tension will slowly even out with the rest of the string, changing the pitch. Heavy bends, (or tremolo action) will have the same result.

... I just thought of something else. Did you just install a new set of strings? You need to stretch them a bit when you put them on, or they will tend to go flat for a while, until they settle in.

To prevent this, change out one string at a time, starting with the high E. Bring it up to match the pitch of the B string. Then gently pull it up, away from the body of the guitar. Don't yank on it so hard that you break the string, just lift it until you notice it getting tighter. The pitch of the string will drop quite a bit. Re-tune it to B, and pull it some more. The pitch won't drop quite as much this time. Re-tune, and pull it some more, repeating this process until the pitch pretty much stays the same. Then tune the string to proper E pitch and change out the B string. Tune it to match the G string and do the pull-and-retune stuff again. Do this with the rest of the strings. The low E can be tuned to low B by using the high B string as a reference.

I hope this helps.