There could be a number of causes of this problem. Try terminating all tasks running in Windows, except for the Explorer and SysTray tasks. This has resolved the problem for some players. For details on the programs appearing in your Windows task list, you can look them up here.
There are two ways to terminate tasks, running in Windows, on the fly. The simple method is to hold down your keyboard's CTRL+ALT+DEL keys. Windows will present you with a CLOSE PROGRAMS dialog or a menu from which you can click a button to open up a TASK MANAGER window. From there, end all running tasks, other than Explorer and Systray.
Another way to accomplish this is to use a utility, such as PC Magazine's free EndItAll 2 program.
Another possible cause of this problem: if your PC has 128 MBs of RAM or less, read this FAQ item.
In a related matter, the more addons you have, the more memory OFP utilizes to retain information on all those addons. If you're a big addon freak, try moving all unofficial addons out of OFP's ADDONS subdirectory to see if this improves your gameplay performance.