|You haven't stated what make and model of computer you have this card on, or how much ram you have, or whether you're having problems in Windows, or whether this happens only when you're in a game - the lag may be because your system is otherwise inadequate.|
If your fan on the card is working okay, your lag problem may be caused by
- a poor connection of the card in the slot it's in.
Remove the AC power to the case, un-install the card, wipe off it's contacts with a tissue or a soft cloth, or better still, wipe them off with methyl (wood; gas line anti freeze) alcohol, or isopropyl (rubbing) alcohol, or a drop of dish washing detergent in water, wipe then dry, install the card, make sure it;s all the way down in it'ss lot, install it's fastening screw, some AGP and PCI-E X16 slots have a latch you can slide on the inner end to lock down that end of the card, restore AC power, try thecomputer.
- your cpu may be overheating. If the cpu fan/heatsink has mung (dust, lint, etc.) on it, clean it off, but DO NOT use a vaccuum cleaner to do that (they produce a tremendous amount of static electricity when running, and anything connected to them can discharge that to your components) - use canned air, or an air nozzle if you have access to an air compressor, or an artist's brush that can be used in small spaces, etc. It may be difficult to clean the top of the heatsink under the cpu fan - the most likely place to have mung on it - and the bottom side of the cpu fan blades unless you remove the fan. If you have a case fan, clean that too if it needs it.
- your power supply fan may be failing.
"Are aftermarket fans good?"
Yes, if you get one suitable for the card, but your card's only worth about $35 brand new and I doubt you can find one for a video card that costs less than $10 or so - how much are you willing to spend on a fan? .
Or you could simply custom mount a 80 mm case fan that costs you $10 or less so it blows on the heatsink side of the card.