my 91 park avenue buick starts but before it actually starts, it sounds as if it has no power or like it wont start but it drives perfectly fine. What could possibly be wrong?

My 91 park avenue buick starts but sounds like its cranking slow like not enough juice.
The other day it died at an intersection but started right back up, another night while
driving, it jerked as if it wanted to shut down but stayed driving. What the heck could be
the problem???..HELP,!