Toyota Repair: Gas mileage/idle question, bosch platinum, filter belts

Question
2002 Toyota solara V6 3.0
I have the above vehicle (v.) with approx. 100,000 miles. I was running my v. at idle speed recently while we had a cold snap near 5 degrees. After about 20 minutes the vehicle started to shake while the check engine light began to flash. I shut the v. off, let it sit for about 10 min. and then started it again. Same thing! I had to drive about 30 miles back to work and while it was still shaking, about halfway through the trip it smoothed out and the check engine light lighted up constant. When I got to my destination, I used my OBD tool and found that Multi injector 1 & 2 had a failure. I cleared it. This was about 3 weeks ago and the v. has been running fine except... I added premium fuel instead of my normal 87 octane and a bottle of injector cleaner after this occurred. This issue prompted me to due a long overdue tune up. I replaced the fuel filter, belts, trans. fluid and spark plugs. I did a continuity check on the wires and also checked them at night to find no visible/electronic problems. With me so far? After changing the spark plugs (Bosch Platinum)which only had one electrode seemed to degrade my gas mileage. I used to normally get 400-420 miles to an 18 gallon tank. Since this issue and spark plug replacement I am only getting 280-300 miles to the tank. That is a 25% loss! I looked into the spark plug matter a little deeper and found that I needed split electrode platinum sparkplugs. I got an exact replacement which was also a Toyota genuine part. I replace the plugs this evening and filled the tank at my normal gas station with 87 octane. After all of this I guess my question is: Could the improper plugs cause a significant fuel loss? And also from the multi injector problem mentioned earlier, could that cause the idle to drop down to 500 RPM?  

Answer
There have been some problems with the ignition coil packs causing a misfire condition, in order to be of any help at all I would need to know what the trouble code(s) were at the time they were retreived, I need the DTC number not the description. The spark plug issue may be irrelevant if there is a misfire but in any case I would not recommend any aftermarket spark plugs such as the bosch plugs as they can cause problems. Whenever the check engine light is flashing it indicates a severe misfire condition that can cause damage to the catalityc converter, is the engine running normally at this time without the check engine light?