The original polyfuses were 11A and the main upgrades move to a 14A replacement. I am not sure how it relates to the MOSFET change but I believe the 14A should work especially when it is in addition to a fan. As far as what to do next, what this fix did was to allow enough current to flow to keep the bed operating if it got too warm it would simply shut down BUT would start working again after it cooled off. If you are seeing a flat line in the heater at the onset then you have another issue going on I would check the wiring itself make sure nothign came loose as a completely dead heat bed is not too common. Remember though you should never see a reading of 0 degrees temp as that means you have a disconnected thermistor for whatever reason. The thermistor when correctly connected and working will always show the air temp so unless it just happens to 0 degrees out side you have another problem. A shorted thermistor will show a very high temp in error and one not connected at all will show zero. This should give you some good starting points.