camaro runs fine unless i dont drive it everyday?

Typically my camaro, 1993 z28 180K miles, runs fine. However if I dont drive it one day when I go to drive it the next it won't run well at all. I have trouble getting it to fire up, and when I do the rpm will be erratic and it will backfire and die. Normally once I get it going and it warms up, all the problems go away again. However I had let it sit for about a week, and when I went to drive it, it did the normal erractic RPM, backfiring etc. Well I got it going and I went and put some gas in it. Well when I went to start it up again it didn't want to start up again. which thats kind of weird. Usually once it starts once it will start again fine. Well I finally get it going and its running fine, so im heading home and and the tach is all over the place and its jerking. I'm having a hell of a time keeping it running. So I pull over and it just quits. I can't get it to turn over at all. I had to have it towed the rest of the way home. Next day I go out with my dad to see if we can figure out whats wrong with it and it starts up like nothing happened.

Any ideas whats causing that?

Like i said its only when I haven't driven it in more than a day. If I drive it everyday its fine.

Gremlins!
Start by checking the simple things..plugs (look carefully for cracked ceramics). Wires and their connection points. Check/clean all sensor connections. Then ad some STP gas treatment and look for any holes or cracks at the tank to filler neck connection that might let water in.

have 91 rs camaro 3.1 mpfi idles perfectly buit as soon as any trottle is given wants to die pop out plentum bogs down. swapped out ecm, replaced o2, swapped out coil, replaced iac, replaced tps, replaced air intake temp senser all still no change any ideas?