stopping a door warping

[tireetim]

I've recently replaced an interior door (big old 50mm thick thing that was warped) with a new cheap moulded door. It's between a hot living room and a cold porch. Door was primed and has 2 coats of paint on to stop any moisture getting in. It seems that this door has now warped too, I suspect due to the difference in temperature between the two rooms. It now catches on the frame unless you push it from one side whilst closing it. Is there any way to avoid this happening or to fix the problem (it gets better sometimes but definitely worse if I light the stove in the living room and make it really warm). The door currently has two hinges but I was thinking about putting a third one in the middle which might help but isn't really a cure.
Any suggestions or are there any doors that I can get that won't warp like this?
Thanks.

 

[big-all]

have you painted both sides and especial attention to top and bottom off the door

 

[tireetim]

Yes, all round. 1 coat of primer then two coats of satin.

 

[big-all]

sounds like you need a better quality door
is this door classified as a security door for insurance purposes

 

[tireetim]

No, it's just a standard interior door (Jewsons). I can't see any other moulded door being any better though. The last one was solid wood and it had warped but it had probably been there for many years. Is there any type of door that is least lightly to go the same way?

 

[Harbourwoodwork]

I like the sound of the old door ,post some pic's something isn't adding up here