Are you sure you have the firing order right? If not the plugs will be
firing between intake and exhaust, which will not give you any power.
(Quick and easy check: switch 2 and 3)

Another thing to check is valve lift. If your cam has gone south it might
do what you described. Easiest way is to take off the valve cover and see
how far down each valve spring retainer goes when the valve opens.
Should be at least 3/8".

This could happen if your cam and lifters are old and especially if the
lifters have been switched or resurfaced. (Ask me how I know....)
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma