it dosen't jump to some "random" proportions, as you stated, buth rathe to the orriginal proportions of the seledtion. my guess is you're making a random/aproximate sellection, adjusting it with "moove selection", then trying to resize it with "moove selected pixels" (or "moove selection", same problem occurs), in which case a Shift+Click on a corner thumb will make your selection (and whatever is in it) jump back to its orriginal proportions (the unadjusted sellection).
to avoid this, i suggest one of the following methods:
a. make your first sellection in such a way that it dosen't need adjusting;
b. adjust with an aditive sellection, insted of "moove sellection";
or
c. after you've adjusted your sellection, copy whatever is in it to a new layer, sellect it again with the wand (invert sellection may be needed) and resize it there
hope this helpes, and if i'm wrong u can go ahead and slapp me over the head