I have also had this problem on two 16c74 (not 74A) chips.
When new they took 10 mins to erase but now they take oner 1 hour.
Microchip claim it is the eraser and the uv bulbs at £20 each need to be
replaced every few hours.
Personally, I think they are passing off the blame but I have found a
solution.  I have been using an industrial turbine to blast the eraser with
cool air while it erases, keeping the chips cool.  I had noticed that the
chips were heating up and this gave me the idea.  My erase time is now under
30 mins with the same chips.  Whatever it does it certainly helps - try a
hairdryer on cold setting, my turbine is only slightly more powerful.  I
would be very interested to know how the devices erase - to heat generates
by the light or by light alone.

Hope this helps - some comment from microchip would be nice

Note: this message is totally serious unlike some I have recently seen about
16c84 code protection.
      Try it.  It might help.

Tim


A happy Christmas to all who read this.