The base timing will not be correct if the TPS is faulty. I recently just went threw something similar. Mine is 1989

I cannot figure out how you passed smog if the timing was incorrect since my smog test was an automatic failure for being 15 degrees too advanced and it too ran great with the timing advanced however it would not pass smog because of the timing being advanced which produced High N0x gases. The Van failed on incorrect timing and High N0x. Hydrocarbons were actually really good

In order to pass smog they had to retard the timing to 8 degrees BTDC which is in spec, however my Van ran like how you described your van, low power, except mine ran like crap with the timing set correctly in my case.

In order for my Van to run well the timing has to be advanced 15 degrees and so I suspect there is something with the distributor, but since the workaround has been advancing the timing I will stick with it as I am getting no detonation , no pinging and no noises. When its time for smog again, I just retard the timing again