I saw something very similar to this a few years back on a early 104 in a CE. The EGR valve was getting stuck open when the car did a extended run at highway speed. Slow down and BOOM! the thing would hardly run, huge internal vacumm leak. Cats glowing, etc... Once the manifold cooled and the EGR valve cooled it would snap shut and run perfect. This was a dumb luck diagnosis as the thing came in "hot" (to borrow your terms) and I happened to smack the valve with a hammer and it instanly straightened out. (jap sleds had a lot of issues with egr's, picked that one up piddeling on mazdas) I disconnected the valve, sent the guy out on a run with a vacum guage on the switchover valve (to double check the venting on decel) and ended up replacing the valve.
Of course the later 104's do the exact oposite and plug the egr tubes. LOL
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.