Have to hold the handle down to finish the flush otherwise the flapper falls to early

Hello Almaden,
Is this a new problem, have you replaced the flapper lately?
In both cases check the following first check the chain from flapper to the handle it should have ¼ of play with flapper seated, Too much play and the flapper doesn't get pulled back enough giving you a short flush and closes quick, too tight the flapper will not seat and you will have water leaking through the flush valve all the time, If you have installed a new flapper did you cut the ring and installed the flapper to the hooks on the overflow tube see the image below.
Check the about if it will solve your problem if not come back, Thanks.

John