Unfortunately, there is probably not much you can do without spending a lot of money on modifications, like adding a turbo or a supercharger. And at 240,000 miles, you have had fabulous service from this engine! It is probably simply tired. If you have not done much engine maintenance recently, I suggest you try all the basics like a thorough fluid change, replacing all the tune-up items like spark plugs, and maybe install new rubber hoses and vacuum lines everywhere. This may help a bit. Then enjoy the car as it gracefully enters old age. (P.S., you probably know that the auto trans is a major power sink compared to the manual transmission, but it's too late to do anything now about it.)