There could be a lot of causes for this. First of all, OFP: Resistance offers much greater graphic detail than the previous OFP version, even when playing old missions and campaigns on the old island. As a result, the recommended system requirements have been upped a notch or two for OFP: Resistance.
If you're new to OFP altogether, I suggest that you search The FAQ for words such as "performance", "tweak", "lag" and "stutter" and that you make sure that all of your motherboard, BIOS, video and sound drivers are up to date.
Even if you've got a relatively fast system with a good video adapter, players have reported stutter and slow performance after they initially installed OFP: Resistance. Some players found the cure was to decrease the new TERRAIN DETAIL and VISIBILITY settings in OFP's VIDEO OPTIONS menu, which they had overenthusiastically maximized to see what Resistance can do.
Some people found that disabling the MULTITEXTURING setting in OFP's VIDEO OPTIONS menu reduced stuttering for them.
I personally experienced problems of sluggishness, heavy processor and memory utilization, slow screen rendering and blurred text with many missions that worked like a charm in earlier OFP versions. With the help of BIS' lead programmer, Ondrej Spanel, we discovered that the problem was caused by me having set TERRAIN DETAIL to HIGH. This happened even though I'm running OFP: Resistance on a 2Ghz machine, with 512MB of RAM and a GeForce 4 video card with 64MB of video memory. When I lowered my TERRAIN DETAIL setting to NORMAL, the problem was completely resolved. Ondrej Spanel concurred that anyone experiencing these symptoms, should try lowering the TERRAIN DETAIL setting.
Another thing to consider is getting more RAM memory. I personally recommend a minimum of 384 MB of RAM.
Some players found that removing all non-official addons, islands, missions and campaigns from OFP's subdirectories resolved the stuttering problems. Other players decided to go to the extreme of uninstalling OFP: Resistance and OFP and reinstalling everything from scratch.