Check your throttle position sensor. When mine went bad the timing would retard with increased throttle.

Here's how to check with an Ohm meter and feeler gauges:
Unplug the connector, it's located on the throttle body on the right side as you look at it from the front. There are four terminals. From top to bottom they are:
Vcc
VTA
IDL
E2
The feeler gauge is inserted between the throttle stop screw and stop lever on the opposite side of the throttle body. It should read like this:

Clearance Between Terminals Resistance
0mm - 0in VTA - E2 0.1-0.8kOhm
0.57mm-0.0224in IDL - E2 Less than 2.3kOhm
0.85mm-0.0335in IDL - E2 Infinity
Full Throttle VTA - E2 3.3-10kOhm
Any position Vcc - E2 4-9 kOhm

Or, the throttle position sensor might not be adjusted properly.

Loosen the two screws on the TPS. Insert a feeler gauge of 0.7mm - 0.0276in between the throttle stop and the throttle lever. Connect the meter between IDL and E2 terminals. Turn the sensor clockwise until the ohm meter deflects, and tighten the screws. Recheck continuity between IDL and E2

Clearance Continuity IDL-E2
0.57mm - 0.0224in Continuity
0.85mm - 0.0335in No continuity

I hope this helps.