Its an '89 245 and the thing of it is, it won’t idle properly. It goes up and down. When I give it a little gas it either pops (I can hear it through the air box) or it just hesitates, then catches and seems smooth in the higher RPM range. When idling on its own I can hear it idle uneven, especially through the exhaust, although it does not pop through the exhaust.
So far I’ve:
Changed plugs, cap, rotor and plug wires.
Changed FPR.
Cleaned throttle body.
Installed newly resoldered fuel pump relay.
Recleaned all fuses and contacts associated with fuel (under hood and at fuse block).
Checked AMM for integrity.
Double checked for vacuum leaks.

The other day I took it for a two hour ride to my sisters and back. I filled up just before getting on the highway, my thinking being that there might be a crack in the hose of the in-tank pump. I recall that it hesitated and popped as I was accelerating. I decided to plunge forward and it did fine. Around town while there – fine. Around town for two days after my return – fine.
Around town two days after my return – not.

Could it be the timing belt? It’s been about 70K since it was changed, according to the sticker on its cover.

Many thanks for all input.