Gauge reading bounce up under load

[Jreyes94]

I've finally decided to tackle the electrical bugs in my 944
Whenever i put the system under load- headlights, heater, stereo; some of my gauge reading will bounce up, until the electrical load is taken off.
For instance, when the car is at almost 3/4 temp, and i turn on the heater while the headlights are on- the temp gauge bounces up into the overheating range (though it is not actually overheating).
My oil pressure gauge bounces a little too as well at the same time.

Few repairs i already have done:
0 gauge wire from battery to alt and starter
new battery
new alternator
all chassis ground have been cleaned of corrosion- including corrosion inside the wires

Anyone know what might be the cause of this?

[RRRacing1970]

Ck...your main battery ground wire that bolts to the back of the block ... it may be frayed and shorting out due to being old and hot!
Ron

[Jreyes94]

Originally Posted by RRRacing1970

Ck...your main battery ground wire that bolts to the back of the block ... it may be frayed and shorting out due to being old and hot!
Ron

It was corroded, i've already replaced this with 0 gauge wire too.