new battery and new alternator and car still loses power after short time any ideas what could cause that?

Hello, it's always best to start at the top with checking vehicle systems and components. Never assume a "new" battery and "new" alternator are both working as they should be. First, start by manually charging your battery to a full charge, and then determine if it is capable of holding a load. Once this is verified, the charging system should be checked for power output: peak amperage and voltage generated. If you see a problem with the output of the charging system, it could be related to the wiring and wiring connector at the alternator, rather than the alternator itself. Or, the "new" alternator could be not working properly. If your vehicle will not jump start, it could still be related to the battery and system state of charge. A starter concern, for example, will not cause the engine to lose power and stall. Further, if you have a problem with engine performance, this could be causing your vehicle to die also. Repeated attempts to restart a vehicle that will not start can run your battery down pretty quickly also, depending on conditions.

Recommend having charging system tested by knowledgeable professional regardless of assumptions.. Battery must be fully charged before accurate charging system test results can be gathered.