Not Sure Whats Wrong, 89' manual
 

hey, lately ive been having trouble with my supra, at first it was only idling at 500 rpm, now its at 1500, but the car barely accelerates even when floored, oil pressure is fluctuating as well, could someone tell what could be causing this?
Thanks

also, ive replaced the heater hose, tried to temporarily fix the head gasket using Blue Devil (liquid poured into coolant system, when thermostat is taken out, when hot enough on head gasket, it solidifies-fixing problem temporarily). But i forgot to mention that after i took it for a test drive in which the acceleration was terrible, my dad said flames shot out the muffler, when i looked it was shooting out sparks.........the oil pressure is fluctuating between 30ish when idling and 60 when driving...

could i be we didnt fix the head gasket, or could this be something else?