There was quite a lenghty post on here recently (can't remember who by now.. - may have been Jim Green? ) regarding this subject, and turned out to be the fuselink wire tails that connect to the battery + terminal had gone bad and high-resistance.
However, this was on a Sapphire, so I guess may not be the same arrangement on Escort..
I would be checking the earth leads from battery to chassis, and battery to engine block/bellhousing. Also check the positive wires from the battery for evidence of a similar situation as above.