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you need mentioned your DVR's brand and details,so anyone may help you. Sorry Never hear this brand DVR,cannot help you.

problem had no connection with heat(as you add one fan),it should had connection with DVR's decoder capacity. you need to lower down the resoution of your IP camera,then it should be ok.

you need mentioned your DVR's brand and details,so anyone may help you.

you can check if the foscam and Hikvision IP camera support ONVIF at first. If so,find one NVR support ONVIF should be ok.