91 starts up and runs fine, then starts missing when it warms up.

[BLACK90ZR1]

My 91 has 12,000 miles on it, and lately it has begun to miss after it warms up. It runs great until it warms up, then it starts running bad. Does anyone have any ideas what might be wrong, or where to start looking for the problem.
Fenwick

Update 1/24/10 : I ran the car today for about 30 miles to get it hot, and got the following resistance readings on the injector coils:

#1 1.7 Ohms
#2 13.8 Ohms
#3 7.0 Ohms
#4 13.5 Ohms
#5 14.0 Ohms
#6 13.6 Ohms
#7 10.7 Ohms
#8 7.7 Ohms
I appears that #1, #3, and #8 are bad. I'm supprised at how well the car ran with them. I'm ordering a new set now.
I did not check the Secondaries, because I'm going to change them anyway.