won't stay running when i first start it up

i only drive my '93 gt every other week or so. i have quite a few mods to the car and it drives pretty well after it's been driven for a few blocks. i have an e303 cam, trick flow heads, billet mass air meter, gt40 intake, 65mm throttle body...can't think of any other mods that may affect this rough starting problem.
i have to continually rev the car or it will stall right when i start it up. also when i'm driving it the idle at first seems to stay around a steady 1700-1900 rpms...it will drop within 30 seconds or so to about 900-1000.
i had the mods done on the car years ago done by a pretty good mechanic.

do any of you know anything i can do to fix my stalling out problem or high idle after driving a bit? perhaps i shouldn't expect the car to drive like a stock mustang with the amount of mods i have done to it?

thanks as always for any input.

i'll have to take a look at the fuel pressure at idle...i have a mechanical gauge on the hood.

the throttle position sensor...where and how do i adjust it? what degree of timing should i set it to? i understand the timing varies but i'd like to have some idea. Thanks both for your help!