When you tested the thermistor, did you measure the ohmic value? What did it read?
Does it show room temp when the bed is cold?
Have you got a heat sink on the Mosfet for the bed?
Do you have thick enough wiring going to the heat bed from D8?
You should have a fan blowing constantly over the Ramps board. The fuse can get hot if its not cooled and can shut the heat bed down.
If you switch the bed on in your host software, what happens?
Note:
Some beds I've seen have a solid copper layer on the back which envelopes the solder terminals on the front causing a short circuit. Cure is to drill out the through hole plating.
Does it show room temp when the bed is cold?
Have you got a heat sink on the Mosfet for the bed?
Do you have thick enough wiring going to the heat bed from D8?
You should have a fan blowing constantly over the Ramps board. The fuse can get hot if its not cooled and can shut the heat bed down.
If you switch the bed on in your host software, what happens?
Note:
Some beds I've seen have a solid copper layer on the back which envelopes the solder terminals on the front causing a short circuit. Cure is to drill out the through hole plating.