87 2.8L Loss of engine power
 

I have a 87 V6/2.8L 4X4, which I got last year for my junker run around. I've rebuilt the transmission, corrected the vacuum system, new starter, reduce the high timing. But I have problem which I cannot seem to identify.

After driving for approximately 15-20 minutes my SES comes on and I fill a loss in engine power. After coming to a stop it is slow to get up to speed and will not accelerate as before the SES came on. I pull over and restart and everything is normal. One additional item, the engine temperature rises when this happens.

First I thought, clogged catalytic converter but there is none. Now I am thinking clogged exhaust.

Any ideas or suggestions?