I installed a 700W incandescent dimmer. Lights go dim after about 10 seconds when I turn on.
Incandescent bulbs and halogen bulbs will work with any dimmer, and since you have a 700W dimmer, it is probably not intended for high efficiency LEDs or fluorescents... it sounds like a more simple model.
All dimmers warm up when the lights are on; as a dimmer becomes warmer, resistance increases; that is a pretty universal property of all materials (that I can think of). Increased resistance, means lower voltage supplied to the lamp. The problem may just be the quality of your dimmer. You should make sure the dimmer is not getting hot; if so, you might need to call an electrician.
Another possibility is that the dimmer has been derated... meaning it has or had "fins" that have been broken off (or maybe just cracked/damaged, perhaps during shipment or installation).
If you're dimming LEDs, then some cheaper LED bulbs naturally dim a few seconds after turning on because they are more efficient when cold and quickly begin to reach equilibrium after being turned on.
If that isnt the case, then I'll need more information before being able to properly answer this question.