Hi & welcome to our corner of the world.
Since you didn't mention anything about a SES light I figure it's not throwing any codes, right?
I do believe that on a 91 Marc's chip is a default power always "on"....but I'm not sure if that applies in the case of deleted secondaries. Therefore, have you checked the actual power switch that it may be a source of a short?
The only other thing I can come up with from my limited knowledge would be that upon entering the "closed loop" stage ( you said when it warms up it stumbles ) one of the sensors is / could be sending bad data to the ECM causing a change in the fueling, or the DIS box is getting funky with the spark signal....but without any DTC's I can't have much faith in that answer.
I should shut up now because I have very little experience with secondary delete systems. Sorry I'm not much help.
Tom