I strongly suspect that your problem is that there is a screen on the end of the pickup line in the tank. This was (is?) standard for Sabres. When crud breaks loose, it gets sucked against the screen and inhibits fuel flow and results in significantly reduced power (also manifests itself as surging power).
We removed the screen on both our Sabres (28 & 38) and the problem went away. Both engines exhibited symptoms like yours. On the 38, after removal, we sucked so much crud into the Raycor, that it was like jello when I pulled the filter several months later. The filter has been clean ever since. The rationale is that you have 3 filters and it's far easier to replace them than to clean the screen. I'm not sure how your screen is installed - it may be at the end of the pickup in the tank (as in our 28) or it could be at the top of the pickup at the top of the tank (as in our 38). Either way, it's somewhere in the tank, unless the PO removed it (doubtful). Yank the screen and your problem should go away.
You should have 3 filters on your Sabre - a Raycor (#1), a lift pump (#2), and a final (#3). The last 2 should be monted on the engine itself.
Sabre 38 "Victoria"