Chances are you flubbed the crimp job. If the cable is out-of-spec, it won't perform at high speed. It's the main symptom for a bad cable (works well @ 10, sucks at 100).
It's really not that hard. Try it like this:
Strip about 2" or so of jacket, you'll be cutting it down to ~1/2" or less later, but the extra "tail" makes it easy to organize the wires.
Start with the orange pair, untwist it back to just before the jacket, smooth all the kinks & wrinkles out of each wire, and put the white-orange, then the orange between your left thumb & forefinger. The jacket of the cable should be just touching the bottom of your index finger.
Repeat for the other pairs. When you're done, the pair-order should be orange-white, orange, green-white, BLUE, blue-white, green, white-brown, brown - left-to-right, with the ends up.
Keep holding the wires - pull/stretch & flatten the collection of wires until they look like a colored ribbon - no gaps,
Using diagonal cutters or scissors, whatever, trim off the excess tail just above your index finger - the goal here is to have ~1/2 inch (~1 cm) of flat colored wires exposed. any more than that, and the cable is out of spec.
Pick up the connector with your right hand, clip down.
Insert the loom of flat wires into the end of the connector. After about ~1/8 inch, the wires will engage the channels of the connector. With a little downward pressure (against the floor of the connector) advance the wires into the connector. THE ENDS OF THE WIRES MUST BE VISIBLE AT THE END OF THE CONNECTOR - also referred to as "Eight shiney copper dots"." Once the wires are mostly inside the connector, you can grap and push from the jacket.
The jacket of the cable must be advanced into the connector until it is well under the dimple in the top of the connector. The wires are going to retract a bit back into the jacket, you'll have to hold tight while pushing with the jacket.
Put the connector into the crimper and crimp.
When you're done, you should still see eight shiney copper dots at the end of the connector, the jacket should be captured by the strain-relief of the connector, and all of the contacts should be flat across the top of the connector.
A few more things, FWIW:
If your connectors came with little plastic insters, you must use them, or the crimp will never work (these come with nearly all CAT5e certified connectors).
The wires can have a MAXIMUM of 1/2" untwisted (any pair).
The cable can have a MAXIMUM of 1/2" of wire exposure (out of the jacket, including the part exposed inside the connector)
Solid conductor cables are easier to terminate, but suck as jumpers, since they break easily after manipulation.
Stranded conductor cable are a PITA to terminate, buy a jumper instead, you'll be way ahead of the game.
If your connectors aren't labled as "Cat 5" (or 5e, 6) then they aren't made for data, they're made for phone (voice / analog), and will add to the probability of a sub-optimal cable.
ALL THE RULES must be followed (pair-order, exposure, strain relief, etc) or your cable is going to suck.
With rare exception, there'll be posts following this with " I just put the wires in and it works just fine" or "color order doesn't matter" or statements that most of this stuff doesn't matter - these people should be ignored, since they haven't got a clue about cabling systems. It DOES matter. Do it right, follow the rules.
If you can't / won't follow the rules, just buy the jumpers or hire someone to terminate the jacks - or suffer with glitchy / flakey network, poor performance, and be held to ridicule ...
Good Luck
Scott