Hot Bed not heating correctly

Hey everyone, I have had some good feedback on topics I posted on this forum, so I am hoping that you guys can help me...

I have a Prusa i3 and I'm having some trouble with the hot bed. Every time I heat up the print bed (to 110*C because I print with ABS) it goes up to 50*C and then starts to cool down again...so from what I have read about the issue, it seems that it could be the firmware, but that is improbable because it worked for about 7 months before this began.

Things I've done to fix this:

- I have double checked the connection to the board and power supply (propper supply that was very expensive).
- I detatched the origional wires leading from the bed to the board and attatched new ones all round (it then heated up much faster, but still stopped at about 62*C)

I don't know what else I can do...can you help?

Possibly the PTC fuse has failed. My preference is to take the power directly from the PSU + terminal to the heatbed via an inline fuse (you can get blade fuseholders with 14ga wiring on ebay), and only route the -ve side through the ramps board for switching. That greatly reduces heating on the ramps pcb traces and avoids the PTC fuse which I've always felt was a poor choice.