Figure this out...
 

'89 Formula / 454 mag...
Runs great for about 5 min (until hot), then power drops out. The engine bogs down, and I loose 500-1000 rpm, randomly in and out.. with a small amount of knock. My first thought was a fuel restriction (before I realized it was a hot engine issue), so I looked into the fuel system... replaced filters, cleaned anti-siphon valve, checked lines, replaced pump, and rebuilt carb (which was full of crap). Still had the problem.. so, I check the timing while it was happening. The timing is going from 24'ish to at least -6 when this happens. It also jumps around in between and higher than 24 for a few cylinders (causing the knock). I've already done the ignition pickup, cap, rotor, and coil, so the only thing left was the ignition module. I replaced that today, still no luck. I visually check all the wiring and grounds, and they look ok. Sometimes its bad enough that I can't even get onto plane. Any ideas?

a restrection in the cooling system

If the timing is jumping back and forth, your timing chain could either be heavily worn or even jumped a tooth.

fuel vapor lock?... check fuel pressure

How much side to side play in dist?, may need to pop out dist and look at the drive teeth on the cam and the driven gear on the dist

Not too familiar with that particular set-up... is there a knock sensor on it ?, if so check the knock sensor output, it's an AC generator, higher voltage during a knock

Found a leak in the intake gasket.. haven't replaced it yet, but I'm thinking this has to be it. The extra air would cause it to go lean and explain the knock and loss of power. The pickup on the timing gun must have been loose or something because there is no vacuum advance.. I'll have to double check that.

I had this problem on my 454 (not as bad though, but timing still varied).I put a cam button on, the cam was actually moving more than it should from front to back the button goes on the end of the camshaft and rides aganst the timeing cover. it cured my problem its a cheap fix if you can do the work yourself the part is about 15bucks I think. but you have to pull the balencer off to pull the timing cover. Hope it works

Brad

Does your engine have a knock sensor?

You might want to check the dist. gear. It might be worn also causing your timing to change.

Check your oil pressure once the engine has reached operating temp,engine could have rod or piston knock and the knock sensor may be detecting it and retarding the timing like you have indicated
is happening...seen it happen more than once ...

I think 1989 had Thunderbolt IV ign. There is no vaccum adv. Advance is controlled by a advance module located on the port side exhaust riser. If the ign. timing is jumping around under a constant throttle, you can bet it is either the module or a loose wire between it and the distributor. By the way, where is your base timing set?