> I'm using MPLAB V6.30 and Forest's serial programmer with the latest
> firmware.

A bit of code to look at would be nice ;-)

Have you checked the errata to see if anything you're doing affects
proper EEPROM operation ?

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2045

Specifically

http://ww1.microchip.com/downloads/en/DeviceDoc/80159e.pdf

http://ww1.microchip.com/downloads/en/DeviceDoc/80212b.pdf

Only INTOSC and PORTB appear here but there could be an indirect
link (for example I'm sure there's a funny connection between PortA
analogue and PortB on the 452 that's not documented)

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