The reasons for over heating can be several. The ones I have encountered were incorrect buffer dilution (5X instead of 1X), higher percentage gels being cast (2% instead of 0.5%), power pack settings being changed or different grade agarose being used to prepare the gels. As you mentioned, the earlier were running fine, have you changed the settings since then? 

@Patrik many thanks for the response. the buffers havent changed from the earlier runs that did no have a problem, so yes it most definately is the power pac. I am checking out the settings as we speak 

Voltage can be higher than the necessary one or incorrect dilution of running buffer

you can try these gels..

http://bitesizebio.com/507/faster-gels/

http://bitesizebio.com/25078/faster-even-cooler-dna-gels/

Thanks for the responses guys. Very insightfull. Quick question...So I normally prepare my buffer with Triz base, but this week i used Triz-HCl...could this be a contributing factor?

Yes because you have more ions in the buffer which drive the current (i hope this is proper English). This might be the problem...

220 ml TBE buffer, 30 ml distilled water and 3.2 gm 1.5% agarose powder which will make 1X TBE buffer then covert it to 10X TBE buffer and finally run the gel at 90 or 95 voltage based on base pair of DNA.

@Norbet Thanks! Am preparing a new buffer with Triz base this morning so fingers crossed, otherwise I will be buying a new power pac

Good luck... Or try the faster gels.. They work really well depending on your size..

Hi Tichaona,

I agree with Maathangi Murali and Masood Sepehrimanesh opinions about the Voltage.

Actually, i am running the mini agarose GelRed with 100 V and the wide agarose GelRed with 180 V.

Good luck.

Take a look for the voltage you use as due to ions presence in the media they may generate heat.