My 72 1800E with a b20f engine has been acting strangely lately and I dont like it. Every time I start the car and drive down the road everything seems normal untill the engine just reaches it's normal running temperature. At that time the engine suddenly stops firing on two cilinders. I know that it is two because it's bad enough to cause me to lose all power, and I know that it's the same two missing over and over because it's a very rethmic miss. This was lasting for five seconds each time and them returning to normal, but now it's lasting 10 seconds and longer so I know that it is getting worse. Always just when the ingine reaches it's normal running temperature and just one time per drive. Sometimes when it doesn't seem like it wants to stop I can get it to snap out of it by geting the rpm's up to 3000. A symptom preceeding this problem that I'm having was a miss-like vibration anytime that my engine was running at 2500 rpm's not below and not over. This symptom still persists.

Please help
Thanks'
Thad Gilbert