Engine faulters under load once warm

[psguardian]

I have come upon a well maintained '91 3.1L 5spd with 295k on the orig motor/trans, which has developed an interesting issue.

Some days it will drive fine for 2hrs, others it will start to stumble & cut out under load after 15min on the road. Lately it has been on the short side of the clock. I have noticed also that the fan no-longer kicks on... All fuses are good & no burnt wires can be found.
I have replaced;
Intake Manifold Gaskets
temp sensor/sender (on back side by firewall)
EGR gasket (haven't cleaned but didn't appear clogged)
Throttle Body gasket.

I'm at a loss as to what it could be. I know there is another temp sender/sensor behind the Thermostat housing, I think this is the one that tells the ecm to turn on the fan? It may have gone out recently, hence the fan not kicking on anymore... This can't be the cause of the issue though as the issue pre-dates the potential sensor/sender failure... Any help much appreciated.

~psguardian

(EDITED for clarification)

[psguardian]

Bump.

20 reads & no suggestions? Did I really stump everyone? Oh I hope not, I know there are people smarter then me out here lol.

TIA,
~psguardian

[Money pit Beretta]

Bump again! For better tech help.
Get some HEET and make sure there is no water in your gas. IsoHeet is the best for that. How new is you fuel filter? Lets start with the basics.