I've posted this before, but if you are not sure whether it is a power or ground issue, here is a way to determine that conclusively.
Grab an ohmmeter and proceed as follows.
First, on the engine, check the continuity from each wheel to the others. Note that in some cases, not all wheels are in the ground path (by design) - you need to know that as well. This check verifies that the ground paths within the loco itself are sound. It's also a good idea to check all the pickup rollers with respect to each other, although it is not strictly necessary for this test. Fix any issues before proceeding.
Now run the loco until it stops at the trouble spot, then power off your layout. With the ohmmeter, check the continuity of a grounded wheel (which you verified above) to the ground rail of the track. I use a clip on one lead to the track ground, and then touch the side or rim of the wheel with a normal probe.
If you don't see continuity, you are losing ground. If you see continuity, you've shown that you have ground, and are therefore losing power instead.
