All lights flash, loss of power, check engine remains

Recently, my ETC light came on so I bought a new throttle body and had a friend install it. After driving it for about a week, all the lights on the dash began to blink while the speedometer jumped up and down. The car would not accelerate faster than about thirty mph and the check engine light remained on after all the other lights turned off. I have yet to get the error codes, but when I do I will update this. The friend who installed it suggested it may be a loose wire on the battery. I attempted to tighten the bolts on the rings that attach to the electrodes on the battery but it did not help. Any ideas?