If you wait more than 1 character period between tests of RI, you will lose characters. Move the serial basics (get character, store in ring buffer, set flag if desired, get out) to an ISR where they belong. Testing serial in the main loop has always been and will always be a bad idea. It depends on the other parts of the main loop not growing slow i.e. something depeends - not on the function but - on the implementation of other processes which is very bad practice.

Have fun,

Erik