I called tompsons auto tops in al. this is where i purches the top, and asked them what would cause this proplem. there reply was this, they didn't know.  never head of this before. oh well i guess when the weather gets better here in ohio I'll take a look at the alignment on the mounting points.  I don't know about the chassie flex my doors shut very well, and when driving it i don't feel any vibration in my seat nor the whole car. I will take all sugestions in to factor, and try to figgure this out. anyone out there please give me your opinion on this mater that is if you have had this happen to you or heard of it before.  thanks my vette friends.

Jonney, the only idea I have is that either the bolts, or the threads in the mounts have some sort f damage. Chassis flex could also be a factor, but mine have never loosened, and my car creaks like the dickens over bumps, or pulling into my slanted driveway. But as I said...I have only had the top off of the '85 2-3 times in all of the years I've had it.
My only suggestion is to carefully look at the threads on the bolts, and look for ANY kind of weirdness...
You might try a SMALL amount of lock-tite on the bolt threads.

Is this the factory top, or do you know if it is aftermarket?

JOEL, it's an aftermarket one. i got it from tompsons astro tops. the bolts that lock down the top are new from gm. i never thought of checking out the mounting brackets.  there the original ones that came on the car.  if these check out ok I"M going to check out the alignment and make sure they bottom out. also lock tight makes a yellow thread locker. they told me i can run the bolts in and out with out it locking the bolts permently, or makeing so hard to get them back off. when i go over speed bumps or a rough road my car doesn't sqeek or rattle. i only hear the plastic trim make a little noise.  how would one tell if you had a lot of frame flex? i have checked my front and rear springs, sway bars and bushings, replaced all my u joints and replaced all my rims and tires ,and had the rims and tires that are directional stadic balance on the car.  my doors shut very well my rear hatch shuts very well and my hood shuts well with no misalignment . the gaps on the doors and hood look great. i don't see any rub marks on the body at all. one thing i forgot to mintion is when i tigten my bolts up i just snug them tight. i heard that if you realy tighten the bolts you could possilby crack the top. i don't know how true that is.

I'd have to dig my owner's manual out to see what the recommended tightening procedure/torque is, but I know I have mine cranked down pretty well with the factory tool/ratchet.
Frame/chassis flex is pretty noticeable, especially if you have the roof off. It is actually called "cowl shake"....you can really see the windshield frame move at times. However, you may not notice it at all, even tho the chassis IS flexing. It doesn't really have anything to do with suspension, or bushings, or any of that....it's just that the C4 cars don't have a real frame, with the body bolted to it. It is a "uni-body", for lack of a better term.  And they do flex.

Jonny: I agree with Joel, check and see if all the threads are perfect.  If so, try to tighten it down  a bit harder.  I 've never heard of cracking a top by doing so, bur I crank mine down pretty tight, and have never had any loosen up like you describe.  I take mine off and on a lot, especially in the summer.

My '92 does the same thing. I used Blue lock-tite. Works great and the bolts still come out with a little resistence.

Since you mentioned the top is a replacement, did you adjust the brackets above the windshield that it bolts to? Maybe the bolts aren't centered on the bracket and bottoming properly. You have to remove the visor and plastic cover to get at the 3 bolts that mount the bracket which has limited movement for the top. Not difficult except for the washer that holds the visor spring. If you loosen the bracket bolts, put the top in normal, then tighten the bracket bolts, you should be aligned for the top bolts. Then re-install the plastic cover and the visor.