[LMP]

A few comments....lacking a definite explanation...
...doesn't this light up the CEL?..
TPS is OK, no doubt..this device is mostly used for transients (rapid throttle changes), not really for steady state throttle.
I'd suspect exhaust invasion...leaky EGR valve, leaky gasket or most possibly burnt exhaust valve. IN fact...were it not for the weird access to spark plugs...I would go for a compression test, at least to confirm or clear the valve option.

This can also be related to poor spark quality--spark has more problem going through denser atmosphere, which happens when throttle is more depressed.....and then the only remaining item in the loop is the ignition module...But this should give a CEL