chevy s-15 2 wheel drive v-6 2.8

I need your help, please. I have a 1986 s-15 pickup 2 wheel drive. I have had this truck for 2 years got it with 60,000 original miles on
it. It has run good until last week. I had driven it for about 50 mile no problems. Went back after setting for 4 hours. It had a loud tapping in the engine. It didnt have any type of noise before. I keep an eye on my gages and it didnt over heat and the oil presure stayed the same. Bought it to the garage he said that there was enough oil getting to the valves. So was told that it was ok to drive just stay local. Went 5 miles away and had a hard time to get it home. It would run ok but get to a light you slow down would act like it was going to stall. As you go down the road the transmission would shift alot (automatic transmission). Then the check gages light came on when you would stop and the oil preasure would drop. The light wouldn't come on while you it was moving. It will start now with no problem. But will buck and kick even back fire when you drive it and the transmission will keep shifting. There is no coolent in the oil. It isnt burning oil. Thought it could be the head casket but the garage said no. The only other thing I can think of is the computer. If anyone could help me I would be very greatful Thank you .
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