When looking at PSUs for GPUs you need to meet the requirement both is watts and amps on the 12v rail of the PSU.
Your GTX660 ti needs a minimum of 450w and 24 amps on the 12v rails,it needs to connectors so each connector needs to carry 12 amps.
So simplify:the cables your connect to the GPU to power it up are 12v cables.Each cable on your PSU can carry 16 amps.Then you need another cable to connect to the GPU because it requires 2 pci-e 6 pin connectors.
Are you using the same Pci-e cable to connect to both the connectors on the GPU?If yes than there is the problem.Since one(1) cable can only carry 16 amps and your gpu need 24 you are staving the gpu for power.
You need 2 separe cable to come from the PSU can connect each one to 1 GPU connector,don't use the same cable for both the connections.
Another thing that could be happening is because the PSU is very bad it does not provide the stated 16 amps per rail so not enough power reaches your GPU for high res such as 1080p or above but may be enough for lower res.
Here is what I would do:
-Buy a good quality PSU for your system for this list:
http://www.eggxpert.com/forums/thread/323050.aspx
If it malfunctions the PSU can kill everything one your system.Don't overlook the PSU and look at it as the cheapest item on the system because it shouldnt be.You should spend quite a bit of money there to assure system stability and safety.
-Connect the PCI-E cable to your GPU and it will work,probably.
Long story short,your PSU has to go!Its is very bad and can burn your whole system!Think about it,60$ for a good PSU or save 40$ and risk having your system fry?Answer is clear